Is it your van? If so, bring it in for 'maintenance' for a day, and install a tracker. Let them know it's been done, and see what happens.
Are they paid day rate, or hourly? If day rate, flip them over to hourly, combined with the tracker above will keep a closer eye. If on hourly, and their worksheets claim they are working more hours than they actually did is a good reason to terminate with immediate effect.