How to detect if an email is a Google Account?

This isn't a complete solution, but you can tell if someone is on Google Apps by looking at their domain's MX records. The link in the answer didn't work for me, so that may be a better solution.

If you're on a Mac, open Terminal and enter $ host {example.com} to determine if their email is hosted by Google.

For example:

$ host yelp.com
yelp.com has address 104.16.57.23
yelp.com has address 104.16.56.23
yelp.com mail is handled by 1 ASPMX.L.GOOGLE.com.
yelp.com mail is handled by 10 ASPMX2.GOOGLEMAIL.com.
yelp.com mail is handled by 10 ASPMX5.GOOGLEMAIL.com.
yelp.com mail is handled by 10 ASPMX3.GOOGLEMAIL.com.
yelp.com mail is handled by 5 ALT2.ASPMX.L.GOOGLE.com.
yelp.com mail is handled by 10 ASPMX4.GOOGLEMAIL.com.
yelp.com mail is handled by 5 ALT1.ASPMX.L.GOOGLE.com.