It's possible to setup a system such that your devs need to work on a server you own, and that logs their activity... but that's probably more trouble than it's worth, and it would likely hinder productivity more than help. Beyond that, it would probably take a lot of your time to take whatever data you've gathered and tie it back to what they said they were doing at any given time, double-checking their hours.
But ultimately, if you're not going to track all of their activity, you'll need to take them at their word. I've had remote developers take advantage of the situation, and it sucks. However, if that's the case, eventually one of two things will happen: 1) They won't get the work done on time because they'll push and see what they can get away with. 2) The amount of work that actually gets done won't justify what they're billing.
Generally, when I'm managing remote developers, if they get their stuff done on time and they stay on budget -- I actually don't care a whole lot if I know exactly how much time they're spending. What matters are results.
Good luck.