I don't think time tracking will help. Best case is you find they were lying about the hours and just get more hours in. Worst case is they were truthful and incompetent.

Either way, it might be best to just fire anyone who bills you 50 hours and works 0. As a contractor, I'd do the opposite - work 40 hours and bill 30, not including some periods of zoning out. My clients would insist that I round it up, not down.

I think you'll find plenty of contractors who suit your more trusting style.