How does a DIY check that brake caliper & pistons move freely.
It's easy. Remove calipers from their mount brackets and support them so the flex hose is not strained. Have helper step on brake pedal and observe distance the piston(s) moves out. Both pistons on both sides ought to be the same. And, ideally, they should retract ever so slightly once the pedal is raised back up again.
Unless the pads are worn to a skin, don't worry about the pistons being accidentally ejected in your test. It will take a number of pedal presses before that even remotely becomes a danger.