How do I know mechanic used new rotors instead of resurfacing them

Couple of ways.. One.. Mechanic TOLD you they are new!!! .. Two, measure rotor thickness...

I would measure the thickness with a dial calipers, your old rotors would have been cut down from the original thickness and therefore be considerably thinner. Also, brand new rotors come with "oil" on them (so they don't rust)and should not be rusty in the "vent slots" around the perimeter. If you just had the work done, they shouldn't be rusty yet. Unfortunately, just because the mechanic told you he replaced them is not always the truth in todays world.