user5935: I have an Ethereum wallet and wish to check if some password was used to create it.
How this could be done?

user6441: You can use the Parity Ethereum Wallet, it has a test password feature: parity.io. You can either use the parity_testPassword RPC call:
curl --data '{"method":"parity_testPassword","params":["0x407d73d8a49eeb85d32cf465507dd71d507100c1","hunter2"],"id":1,"jsonrpc":"2.0"}' -H "Content-Type: application/json" -X POST localhost:8545

Or use the graphical wallet interface and go to Accounts > Open the Account in question > Click password and chose "Test".
Disclosure: I work for Parity.

user5935: It is possible with pyethsaletool:
./pyethsaletool recover -w your-wallet.json -p password

In case of password failure it will throw something like 
ValueError: String ending with 'X' can't be PCKS7-padded

If password is valid, it will show your seed (long string of random characters).
Actually pyethsaletool uses supplied password to AES-decrypt encseed field from the wallet. This seed is SHA3-hashed to get private key and ethereum address which is compared to the one from the wallet.

user50029: You could use MyEtherWallet.com to check your password. Go to the "View Wallet Info" section and select your keystore file (the wallet file with .json at the end). Enter the right password. If it shows "Success! Here are your wallet details." you have the right password.

user24905: You could check by starting a geth console and using the personal.unlockAccount function: 
$ geth console
> personal.unlockAccount("address", "password")


5935: <p>I have an Ethereum wallet and wish to check if some password was used to create it.<br />
How this could be done?</p>
 6441: <p>You can use the Parity Ethereum Wallet, it has a <em>test password</em> feature: <a href="https://parity.io" rel="nofollow noreferrer">parity.io</a>. You can either use the <code>parity_testPassword</code> <a href="https://github.com/paritytech/parity/wiki/JSONRPC-parity_accounts-module#parity_testpassword" rel="nofollow noreferrer">RPC call</a>:</p>

<pre><code>curl --data '{"method":"parity_testPassword","params":["0x407d73d8a49eeb85d32cf465507dd71d507100c1","hunter2"],"id":1,"jsonrpc":"2.0"}' -H "Content-Type: application/json" -X POST localhost:8545
</code></pre>

<p>Or use the graphical wallet interface and go to Accounts > Open the Account in question > Click password and chose "Test".</p>

<p><sup><em>Disclosure: I work for Parity.</em></sup></p>
 5935: <p>It is possible with <a href="https://github.com/ethereum/pyethsaletool/blob/master/pyethsaletool.py" rel="nofollow">pyethsaletool</a>:</p>

<pre><code>./pyethsaletool recover -w your-wallet.json -p password
</code></pre>

<p>In case of password failure it will throw something like </p>

<pre><code>ValueError: String ending with 'X' can't be PCKS7-padded
</code></pre>

<p>If password is valid, it will show your seed (long string of random characters).</p>

<p>Actually <code>pyethsaletool</code> uses supplied password to AES-decrypt <code>encseed</code> field from the wallet. This seed is SHA3-hashed to get private key and ethereum address which is compared to the one from the wallet.</p>
 50029: <p>You could use MyEtherWallet.com to check your password. Go to the "<a href="https://www.myetherwallet.com/#view-wallet-info" rel="nofollow noreferrer">View Wallet Info</a>" section and select your keystore file (the wallet file with .json at the end). Enter the right password. If it shows "Success! Here are your wallet details." you have the right password.</p>
 24905: <p>You could check by starting a <a href="https://github.com/ethereum/go-ethereum/wiki/geth" rel="noreferrer">geth console</a> and using the <a href="https://github.com/ethereum/go-ethereum/wiki/JavaScript-Console#personalunlockaccount" rel="noreferrer">personal.unlockAccount</a> function: </p>

<pre><code>$ geth console
&gt; personal.unlockAccount("address", "password")
</code></pre>
