I read several suggestions but none of them would stop a pro carder, still it's ok for newbies.
I would recommend anyway to:
- compare country and state of signup ip with country and state of signup form
- compare join ip location with logins ip location
- compare country of credit card bin with country of ip and country on join form
- validate email address
- validate phone number using online phone books
Again this won't stop a pro carder but still better than nothing.
My 2 cents
One good free tool for manual lookups is this:
http://www.dnsstuff.com/pages/merchant.htm