Hi,
As a SMF owner, that first listed criteria won't work (1/2 of my tube geek members use gmail, some eastern ones use mail.ru)
Admin approval is the only tried and true way. Run the username and/or IP address through Google. If they don't show up on stopforumspam.com or Project Honeypot and Google doesn't return 1,000+ results for the IP or username, good chance it's legit :-)
Just my twonie.
Cheers!