[Novalug] How can script know if it's been 'sourced' rather than 'called'?

[Ben Creitz]

I have little script which starts ssh-agent for a given period of time
and loads a private key.  The script must be "sourced" with source or
. in order to work, so that the calling shell can access the env vars
that ssh-agent exports.

Is there a way for the script to fail and echo proper usage if
somebody calls it directly?  Of course, I can remove the execute bit.
But I would like to avoid people calling it with

  bash start-ssh-agent 5

, too.  It really isn't too important, but I am curious to see if
there's an answer out there.

Ben