Get them to use an online time tracking/project management/invoicing application. I recommend FreshBooks, partially because I work for them and partially because they have a built in contractor section. If you connect your RSS reader to the homepage feed it should let you know when people are submitting hours.

Also, it should be free for you if you just have one project and are only using contractors. It is true that they could just leave it on for a while, but if they have to submit notes while they track time, it would be a pretty big pain in the arse for them to constantly fake things.