user15492: When looking at Bitcoin unconfirmed transactions, I'm trying to distinguish between txs that are likely to confirm (whether that be in the first next block or in 2 hours) versus txs that are likely to be replaced, or never to confirm at all.
Obviously, txs with the opt-in RBF flag fall in the latter category. 
But I guess there are other criteria, such as:

having an extremely low fee, or even zero fee. 
Having lots of dust outputs. 
Depending on unconfirmed inputs.

Anything else I should take into consideration? What other factors could make a tx less likely to end up confirmed? 
For example, besides dust output, do dust (i.e. many small) inputs make a tx less sure to confirm? (other than more inputs causing larger tx data, thus resulting in a lower fee when measured in satoshis per KB)

user3697: Replacing transactions without confirmation is relatively easy.
I would tag every transaction without confirmation as "suspicious".
RBF is a policy and miners could also replace transaction that do not sign opt-in-RBF.
Best way to check if a 0-conf transactions will be mined soon:

check nSequence number (only accept 0-conf if nSequence number is INT_MAX)
check the transactions fee-rate and compare it with your estimated fee (requires a mempool/bitcoin-core)
Calculate the risks: every 0-conf transaction can be replaced/double-spent! High value items should not be "dispatched" < 6 confirmations (IMO).


15492: <p>When looking at Bitcoin unconfirmed transactions, I'm trying to distinguish between txs that are likely to confirm (whether that be in the first next block or in 2 hours) versus txs that are likely to be replaced, or never to confirm at all.</p>

<p>Obviously, txs with the opt-in RBF flag fall in the latter category. </p>

<p>But I guess there are other criteria, such as:</p>

<ul>
<li>having an extremely low fee, or even zero fee. </li>
<li>Having lots of dust outputs. </li>
<li>Depending on unconfirmed inputs.</li>
</ul>

<p><strong>Anything else I should take into consideration?</strong> What other factors could make a tx less likely to end up confirmed? </p>

<p>For example, besides dust output, do dust (i.e. many small) <em>inputs</em> make a tx less sure to confirm? (other than more inputs causing larger tx data, thus resulting in a lower fee when measured in satoshis per KB)</p>
 3697: <p>Replacing transactions without confirmation is relatively easy.
I would tag every transaction without confirmation as "suspicious".</p>

<p>RBF is a policy and miners could also replace transaction that do <strong>not</strong> sign opt-in-RBF.</p>

<p>Best way to check if a 0-conf transactions will be mined soon:</p>

<ul>
<li>check nSequence number (only accept 0-conf if nSequence number is INT_MAX)</li>
<li>check the transactions fee-rate and compare it with your estimated fee (requires a mempool/bitcoin-core)</li>
<li>Calculate the risks: every 0-conf transaction can be replaced/double-spent! High value items should not be "dispatched" &lt; 6 confirmations (IMO).</li>
</ul>
