user135893: Looking at these two transactions for example: tx1 and tx2. tx1 is an OP_RETURN trasaction, tx2 is a normal transaction.
raw hex from tx1 (OP_RETURN): 0100000000010173a9eafb38a83662d28a5c885165c7aa3f27694e39f0ba4e4fc86d98ff3dd2d60000000000ffffffff0200000000000000004b6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba2a940600000000002251203e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb9901407ac1f8560a33867cdfa3ad136488ed7f4f5d1f4fc6a32822d8dd6c5e47e9317f0f2a4381349a803d223773711d6df8303e9c23e60782633450c735b2275d9a2100000000
raw hex from tx2:
0200000000010191fd88fc50bd8932c8da649e0c54b42ab3598a89c420c28bf6fad2d443711dbd2e00000000ffffffff016bef0c000000000017a9140afdb9b53981bfac9c53d01f6389b5ebecf8499f8702483045022100f0d0944b835095b5fcad86df999831ae3eade37c0eb68649b83a55ca5f1854af0220231561ec647b7bd9179c5f809e878a4cfd09818514b42ca8a274b11dfaa3a3ed012103e5d3e103615a36046dfbe880d6fb4c47e5227d8176635310c5dd08d5aa43f99c00000000
What am I looking for?

user30668: You can use bitcoin-cli to decode the transaction and get a sense of what is going on:
bitcoin-cli decoderawtransaction 0100000000010173...275d9a2100000000

That returns:
{
  "txid": "00fddd900b735a88ab0ef5eeaa3efbc869d4e52493da8a9467ec782355cb39ff",
  "hash": "b4c4817e908d9ba317e9b1a08180b51c0ce77adca946e7f894560fb07447acf8",
  "version": 1,
  "size": 246,
  "vsize": 195,
  "weight": 780,
  "locktime": 0,
  "vin": [
    {
      "txid": "d6d23dff986dc84f4ebaf0394e69273faac76551885c8ad26236a838fbeaa973",
      "vout": 0,
      "scriptSig": {
        "asm": "",
        "hex": ""
      },
      "txinwitness": [
        "7ac1f8560a33867cdfa3ad136488ed7f4f5d1f4fc6a32822d8dd6c5e47e9317f0f2a4381349a803d223773711d6df8303e9c23e60782633450c735b2275d9a21"
      ],
      "sequence": 4294967295
    }
  ],
  "vout": [
    {
      "value": 0.00000000,
      "n": 0,
      "scriptPubKey": {
        "asm": "OP_RETURN 4575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba",
        "desc": "raw(6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba)#vxme4rep",
        "hex": "6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba",
        "type": "nulldata"
      }
    },
    {
      "value": 0.00431146,
      "n": 1,
      "scriptPubKey": {
        "asm": "1 3e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb99",
        "desc": "rawtr(3e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb99)#nkpjq33l",
        "hex": "51203e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb99",
        "address": "bcrt1p8exfw8kd8tgzpqeuh49jan037g5agvxl5j9dcvsr7hmeeqqalwvs884ffw",
        "type": "witness_v1_taproot"
      }
    }
  ]
}

You can see how, in the output section of the transaction, the first output is an OP_RETURN  output, and that the data associated to it is:
4575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba
Also, notice how the OP_RETURN opcode is 0x6a (you can check the bitcoin wiki for that).
Therefore, the piece you're looking for inside the hex transaction is:
6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba

You may also notice there is an additional byte of information between the OP_RETURN opocode and the data: 0x49. That's the size of the associated data in bytes (encoded in hex). 49 hex equals 73 dec, so the length is 73 bytes or 146 hex characters.

135893: <p>Looking at these two transactions for example: <a href="https://mempool.space/tx/00fddd900b735a88ab0ef5eeaa3efbc869d4e52493da8a9467ec782355cb39ff" rel="nofollow noreferrer">tx1</a> and <a href="https://mempool.space/tx/c805681da0216e7fb0efe3a574eb2b24872c04f66e99559c7ce72d74484f1d4e" rel="nofollow noreferrer">tx2</a>. tx1 is an OP_RETURN trasaction, tx2 is a normal transaction.</p>
<p>raw hex from tx1 (OP_RETURN): <code>0100000000010173a9eafb38a83662d28a5c885165c7aa3f27694e39f0ba4e4fc86d98ff3dd2d60000000000ffffffff0200000000000000004b6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba2a940600000000002251203e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb9901407ac1f8560a33867cdfa3ad136488ed7f4f5d1f4fc6a32822d8dd6c5e47e9317f0f2a4381349a803d223773711d6df8303e9c23e60782633450c735b2275d9a2100000000</code></p>
<p>raw hex from tx2:
<code>0200000000010191fd88fc50bd8932c8da649e0c54b42ab3598a89c420c28bf6fad2d443711dbd2e00000000ffffffff016bef0c000000000017a9140afdb9b53981bfac9c53d01f6389b5ebecf8499f8702483045022100f0d0944b835095b5fcad86df999831ae3eade37c0eb68649b83a55ca5f1854af0220231561ec647b7bd9179c5f809e878a4cfd09818514b42ca8a274b11dfaa3a3ed012103e5d3e103615a36046dfbe880d6fb4c47e5227d8176635310c5dd08d5aa43f99c00000000</code></p>
<p>What am I looking for?</p>
 30668: <p>You can use <code>bitcoin-cli</code> to decode the transaction and get a sense of what is going on:</p>
<pre><code>bitcoin-cli decoderawtransaction 0100000000010173...275d9a2100000000
</code></pre>
<p>That returns:</p>
<pre><code>{
  &quot;txid&quot;: &quot;00fddd900b735a88ab0ef5eeaa3efbc869d4e52493da8a9467ec782355cb39ff&quot;,
  &quot;hash&quot;: &quot;b4c4817e908d9ba317e9b1a08180b51c0ce77adca946e7f894560fb07447acf8&quot;,
  &quot;version&quot;: 1,
  &quot;size&quot;: 246,
  &quot;vsize&quot;: 195,
  &quot;weight&quot;: 780,
  &quot;locktime&quot;: 0,
  &quot;vin&quot;: [
    {
      &quot;txid&quot;: &quot;d6d23dff986dc84f4ebaf0394e69273faac76551885c8ad26236a838fbeaa973&quot;,
      &quot;vout&quot;: 0,
      &quot;scriptSig&quot;: {
        &quot;asm&quot;: &quot;&quot;,
        &quot;hex&quot;: &quot;&quot;
      },
      &quot;txinwitness&quot;: [
        &quot;7ac1f8560a33867cdfa3ad136488ed7f4f5d1f4fc6a32822d8dd6c5e47e9317f0f2a4381349a803d223773711d6df8303e9c23e60782633450c735b2275d9a21&quot;
      ],
      &quot;sequence&quot;: 4294967295
    }
  ],
  &quot;vout&quot;: [
    {
      &quot;value&quot;: 0.00000000,
      &quot;n&quot;: 0,
      &quot;scriptPubKey&quot;: {
        &quot;asm&quot;: &quot;OP_RETURN 4575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba&quot;,
        &quot;desc&quot;: &quot;raw(6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba)#vxme4rep&quot;,
        &quot;hex&quot;: &quot;6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba&quot;,
        &quot;type&quot;: &quot;nulldata&quot;
      }
    },
    {
      &quot;value&quot;: 0.00431146,
      &quot;n&quot;: 1,
      &quot;scriptPubKey&quot;: {
        &quot;asm&quot;: &quot;1 3e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb99&quot;,
        &quot;desc&quot;: &quot;rawtr(3e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb99)#nkpjq33l&quot;,
        &quot;hex&quot;: &quot;51203e4c971ecd3ad020833cbd4b2ecdf1f229d430dfa48adc3203f5f79c801dfb99&quot;,
        &quot;address&quot;: &quot;bcrt1p8exfw8kd8tgzpqeuh49jan037g5agvxl5j9dcvsr7hmeeqqalwvs884ffw&quot;,
        &quot;type&quot;: &quot;witness_v1_taproot&quot;
      }
    }
  ]
}
</code></pre>
<p>You can see how, in the output section of the transaction, the first output is an <code>OP_RETURN </code> output, and that the data associated to it is:</p>
<p><code>4575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba</code></p>
<p>Also, notice how the <code>OP_RETURN</code> opcode is <code>0x6a</code> (you can check the <a href="https://en.bitcoin.it/wiki/Script" rel="noreferrer">bitcoin wiki</a> for that).</p>
<p>Therefore, the piece you're looking for inside the hex transaction is:</p>
<pre><code>6a494575726f7065206973207469726564206f66206265696e67207375627365727669656e7420746f207468652055532e204c6f6e67206c6976652052757373696120f09f87b7f09f87ba
</code></pre>
<p>You may also notice there is an additional byte of information between the <code>OP_RETURN</code> opocode and the data: 0x49. That's the size of the associated data in bytes (encoded in hex). 49 hex equals 73 dec, so the length is 73 bytes or 146 hex characters.</p>
