I've never tried to screen people, but I do know someone who asked for a facebook profile in the ad. You could ask for something like that--LJ/myspace/friendster/whatever--and poke through their profile and their friends' profile to get an idea of what they're like. The nice thing about facebook is that it's a limited set of people, which increases the likelihood that someone you know knows them. I don't think outright asking for a character reference would work. Any creep worth their salt can write a sockpuppet reference.