How to tell if a public SSH key has a passphrase

This is probably a noob question, but how can I determine if the public SSH key someone gives me has a passphrase or not?

We have a situation where I am not generating the SSH keys for users, but I want to make sure every SSH key I put on a server has a passphrase, but I get the feeling the passphrase is only part of the private key.

Thanks!

Solution 1:

This is not something you can determine from the public half of the key. Even if you could determine it, what's to stop the user from subsequently removing it?

When you remove the passphrase from the private side of the key, the public side doesn't change.

Solution 2:

You can not tell if a private key has a pass phrase or not.