1) All the usual stuff including but not limited to barrel & muzzle crown, feed ramp, the frame in general for stress or drop cracks.
2) Check the frame where the slide contacts the entire length both sides (remove slide for this inspection). Sometimes the polymer can get burred or roughened.
3) Ejector claw, intact, or worn/chipped?
4) it would be nice to get all the front sight blades & backstraps with it
I'm sure there are other things to look for, but I'll leave that for the others to point out.