remove the brake pads and refit the caliper, it should now be possible to slide the caliper (if it is a sliding caliper) using hand pressure.
usually 4 piston calipers are not sliding type.
with the pistons pushed out a bit (by carefully pressing the brake pedal) it should be possible with strong hand pressure to push each and every piston back in to the caliper. A short lever may be used if you are weak or it is difficult to get a purchase. It is usually easy to tell if any piston is sticking, it will be obvious by the greater amount of pressure needed.
while the pistons are sticking out a bit you can check for rust or damage to the polished outside surface of the piston.
brake shudder is almost invariably caused by warped discs
mrcheerful