It is possible to determine a great deal of information concerning the condition of a car's brake calipers by performing a visual inspection. Cracks or fluid leaks are an indicator the unit needs replacing. With the tires of the vehicle off the ground, when the brakes are applied, the caliper should move and the rotor should not.
Air in the brake lines interferes with the operation of calipers. With a brake bleeder jar half-filled with clean fluid, open the bleeder valve while an assistant applies the brakes. Air bubbles in the jar indicate a problem; it is necessary to troubleshoot the issue and take steps to eliminate the air in the system. Air in the brake lines sometimes indicates a problem with the caliper, an issue requiring a replacement.
Occasionally, a caliper does not float properly due to damaged pins or a need for lubrication. If brake pads try to move when the brakes are applied, remove the pins, then clean and relubricate them. If this does not fix the brakes, a replacement is in order.
If applying the brakes does not move the caliper, the problem is likely with the piston. Autos.com recommends installing a new caliper as a unit if the test reveals a bad piston.