Maxmind is a great tool but only gets about 80% of the scammers. We always manually verify orders on top of maxmind, but maxmind definitely makes it easier by getting rid of most of em.

Proxys are a given, look at the IP address, if it has a reverse DNS a proxy will usually be obvious compared to a residential or business connection. If it doesn't have a reverse DNS, then use Google to search the IP and see what comes up.