Is it possible to know if a domain is using Google Apps for email?

Dig their MX records:

$> dig domainname.com mx

If they're with Google, you'll know it.

domainname.com.            2198    IN      MX      10 ASPMX.L.GOOGLE.com.
domainname.com.            2198    IN      MX      20 ALT1.ASPMX.L.GOOGLE.com.
domainname.com.            2198    IN      MX      20 ALT2.ASPMX.L.GOOGLE.com.
domainname.com.            2198    IN      MX      30 ASPMX2.GOOGLEMAIL.com.
domainname.com.            2198    IN      MX      30 ASPMX3.GOOGLEMAIL.com.
domainname.com.            2198    IN      MX      30 ASPMX4.GOOGLEMAIL.com.
domainname.com.            2198    IN      MX      30 ASPMX5.GOOGLEMAIL.com.

I think easiest would be just to type it into Google Domain Tester. It does the work for you.

http://www.googledomaintester.com

Cheers!