check for heavy body damage, make sure the drive turns easily, I have seen a fair amount where it will turn then lockup and then click as the hammers move but the motor doesn't.
Check the direction selector doesn't move to freely, most have a roll pin that the knob attaches to the shaft with and a few I have worked on have sheared meaning you can spin the knob round and round.
check for rusty stains around the exhaust where the compressor used wasn't supplying dry air.