I'm no expert at all, and I don't know what damage was sustained, but obvious things are check for colour match with adjacent panels, check for overspray on side-repeaters or black trim etc.
Also, the paint finish quality should be the same as the factory finish. Sometimes, it can be 'orange-pealy' and not as smooth as it should be.
If they've used filler, is it smooth, or are there slight ripples? If a panel has been replaced, is the new one correctly aligned? Are the gaps all the same with adjacent panels.
Looking down the side of the car from the front or back, crouch down, so your eye is at waist level and look for any uneven surfaces or dents that they may have missed.
If you look at the car and can't tell it's had any sort of repair, this is ideal. A lot depends on the colour (silver and pearlescents are very difficult to match) and the age of the car, with older cars suffering from paint fade etc.
Anyway, hope this helps. All the best !