Let's say I have access to the private portion of an RSA key-pair. How can I check if this key has associated passphrase or not?
The keyfile will have a different header if it is password protected.  Here's the top of a key without a passphrase:

-----BEGIN RSA PRIVATE KEY-----
MIIEogIBAAKCAQEA3qKD/4PAc6PMb1yCckTduFl5fA1OpURLR5Z+T4xY1JQt3eTM


And here's the top of a key which is passphrase-protected:

-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: DES-EDE3-CBC,556C1115CDA822F5

AHi/3++6PEIBv4kfpM57McyoSAAaT2ECxNOA5DRKxJQ9pr2D3aUeMBaBfWGrxd/Q


Unfortunately, that only works looking at the files.  I know of no way for a server to be able to tell if the keys being presented to it were protected with a passphrase, which is the most useful place to be able to leverage that sort of info.
