When and if you do find it's frozen, the best thing to do is pull it for a complete rebuild. There are cases where you can (on the cheap) free it up in the car but you'll eventually have a reliability issue if you go that route.
I rebuild 50-year-old hemis and have found some wrecking yard motors with two or three pistons frozen. The best method I've found is to disassemble the motor including all internals except the pistons that are stuck. Then I pour about a quarter to a half inch of penetrating oil on the top of the piston and let it set for several days. When that's done I remove the oil and take a piece of hard wood about an inch thick (don't use soft wood) and cut it to fit roughly in the cylinder. Then I use a 5-pound sledgehammer to hit the hard wood. After about three or four good smacks the piston should be free. Sometimes you don't need to do this at all, just a tap will do. Depends on how badly it's stuck. If the piston won't come loose using this method I put more penetrating oil in and let it sit another week. Eventually it will come loose and shouldn't damage the cylinder.
One exception though. If it's frozen due to rust you may have to break the piston into pieces to get it out. Not something you would bother to do on a SBC Chevy since they're a dime a dozen but on a rare or valuable block sometimes you have to do everything you can to save them. If you have to go this far you probably will also have to sleeve the cylinder