A lot of times we simply have to go on instinct. What I do is send PMs to each account and see how they explain things. You can tell a lot about people by their tone and their use of grammar and syntax---their age, their occupation, even their outlook on life.

It's not a perfect system; some "problem children" slip through and create havoc until they're banned, or they post on some other forum bragging about how they fooled the mods at X site.. But we watch all new members fairly closely, and if they don't cause trouble, we usually let them go on, even if it means a few people get away with multiple accounts (as long as we don't know about it officially).