I'm exchanging emails with a number of different users - is it possible to look at the mail headers and figure out if those users are using Google Apps as the mail server?
Dig their MX records:
If they're with Google, you'll know it.
If you are using .NET, and make the assumption that their domain is MAIL.theirdomain.com, you can do this:
Google Apps accounts that I've seen all end with google.com, such as ghs.google.com.