How to tell if key is encrypted?

[Bob Proulx]

I would like to automatically deduce in a script if an ssh key is
encrypted or not.  Basically in a very particular application I want
to be the BOFH and enforce that users place a passphrase on their
id_rsa key.  If they don't put a passphrase I want to send them back
to ssh-keygen until they do.  I have not been able to deduce a way to
detect this yet.  Any hints?

Thanks
Bob