The Federal Trade Commission has a good video on their website.
It's an interview with a few good, honest people who fell prey to some Work at Home "Biz Op" scams and the scammer who ripped one of them off. Pay close attention to the scammer when he explains how he'd deal with people who'd start asking questions he didn't want to answer. Remember, when someone refuses to give straight forward answers to legitimate questions, it's usually because any honest answer would hurt their sales pitch.
There are also some good consumer information links on that page, please take a look.