How do I verify/check/test/validate my SSH passphrase?

You can verify your SSH key passphrase by attempting to load it into your SSH agent. With OpenSSH this is done via ssh-add.

Once you're done, remember to unload your SSH passphrase from the terminal by running ssh-add -d.

ssh-keygen -y

ssh-keygen -y will prompt you for the passphrase (if there is one).

    If you input the correct passphrase, it will show you the associated public key.
    If you input the wrong passphrase, it will display load failed.
    If the key has no passphrase, it will not prompt you for a passphrase and will immediately show you the associated public key.

e.g.,

Create a new public/private key pair, with or without a passphrase:

$ ssh-keygen -f /tmp/my_key
...

Now see if you can access the key pair:

$ ssh-keygen -y -f /tmp/my_key

Following is an extended example, showing output.

Create a new public/private key pair, with or without a passphrase:

$ ssh-keygen -f /tmp/my_key
Generating public/private rsa key pair.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /tmp/my_key.
Your public key has been saved in /tmp/my_key.pub.
The key fingerprint is:
de:24:1b:64:06:43:ca:76:ba:81:e5:f2:59:3b:81:fe [email protected]
The key's randomart image is:
+--[ RSA 2048]----+
|     .+          |
|   . . o         |
|    = . +        |
|   = + +         |
|  o = o S .      |
|   + = + *       |
|    = o o .      |
|     . .         |
|      E          |
+-----------------+

Attempt to access the key pair by inputting the correct passphrase. Note that the public key will be shown and the exit status ($?) will be 0 to indicate success:

$ ssh-keygen -y -f /tmp/my_key
Enter passphrase:
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDBJhVYDYxXOvcQw0iJTPY64anbwSyzI58hht6xCGJ2gzGUJDIsr1NDQsclka6s0J9TNhUEBBzKvh9nTAYibXwwhIqBwJ6UwWIfA3HY13WS161CUpuKv2A/PrfK0wLFBDBlwP6WjwJNfi4NwxA21GUS/Vcm/SuMwaFid9bM2Ap4wZIahx2fxyJhmHugGUFF9qYI4yRJchaVj7TxEmquCXgVf4RVWnOSs9/MTH8YvH+wHP4WmUzsDI+uaF1SpCyQ1DpazzPWAQPgZv9R8ihOrItLXC1W6TPJkt1CLr/YFpz6vapdola8cRw6g/jTYms00Yxf2hn0/o8ORpQ9qBpcAjJN
$ echo $?
0

Attempt to access the key pair by inputting an incorrect passphrase. Note that the "load failed" error message will be displayed (message may differ depending on OS) and the exit status ($?) will be 1 to indicate an error:

$ ssh-keygen -y -f /tmp/my_key
Enter passphrase:
load failed
$ echo $?
1

Attempt to access a key pair that has no passphrase. Note that there is no prompt for the passphrase, the public key will be displayed, and the exit status ($?) will be 0 to indicate success:

$ ssh-keygen -y -f /tmp/my_key_with_no_passphrase
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDLinxx9T4HE6Brw2CvFacvFrYcOSoQUmwL4Cld4enpg8vEiN8DB2ygrhFtKVo0qMAiGWyqz9gXweXhdmAIsVXqhOJIQvD8FqddA/SMgqM++2M7GxgH68N+0V+ih7EUqf8Hb2PIeubhkQJQGzB3FjYkvRLZqE/oC1Q5nL4B1L1zDQYPSnQKneaRNG/NGIaoVwsy6gcCZeqKHywsXBOHLF4F5nf/JKqfS6ojStvzajf0eyQcUMDVhdxTN/hIfEN/HdYbOxHtwDoerv+9f6h2OUxZny1vRNivZxTa+9Qzcet4tkZWibgLmqRyFeTcWh+nOJn7K3puFB2kKoJ10q31Tq19
$ echo $?
0

Note that the order of arguments is important. -y must come before -f input_keyfile, else you will get the error Too many arguments..

The best answer so far, that I found on the web is:

ssh-keygen -y -P "" -f ~/.ssh/id_rsa

That will attempt to read the private key with the passphrase as empty string. That will show a public key perfectly fine if you have a key with no passphrase.

Otherwise the app will exit with an error code and the message:

Load key "~/.ssh/id_rsa": incorrect passphrase supplied to decrypt private key