I think that part of the scam is that the fake check is drawn on some bank that takes a very unusually long time to return bad checks. You might think that after a couple of weeks if it hasn't bounced it's good, but I think these checks can take a month or longer to bounce.

You could ask for them to send a check and deposit it, just to see how long it takes for it to bounce, and for the bounced deposit charge (don't touch those 'virtual funds' or your bank will be pretty angry later on!) you could get some entertainment value out of their frantic emails trying to get you to send som cash before their check comes back to roost.