Most probably do NOT check. But maybe some do just for giggles, or sometimes if they don't believe you. Or maybe you are borderline and they want to talk to your references to get a better idea. These are all possible... I'm not saying these things actually happen. At the very least, it forces you to be honest, because IF they wanted to check up on you, then they have the means to do so.
My points in all this are:
1. don't lie unless you want to take the risk, however remote
2. don't worry about things where there are lack of appropriate references; it's mostly to keep you honest, not to check up on every activity of every applicant.