I know quite a bit about computers, scripts and web design, and realistically there isn't a "foolproof" method.
Legally I expect all you really have to do is make sure that they accept the board TOS when they join and that those TOS include that they are over 18 years old.
Then if it turns out there' a kid on the site and somebody starts making noises, essentially the child broke the contract they have with you, and they don't have a leg to stand on.
I mean if you look at porn sites, (and who doesn't?!) the ones that even bother to have a buffer page just require you (usually) to click a button which says you accept the terms of use (i.e. you're over the age of 18 and wave any rights etc.).
Realistically, there's nothing much else the webmaster can do. The credit card idea is a good one, but then again, I'm 20 and don't have a credit card [I work on the reasoning that the heads of all major credit card companies don't have one, and if they don't have their own product, why should I?], and even if I did, it'd be a UK credit card, which might not work on a US based check. It probably would, but what about somebody from China, for example?