The first sign that the callipers need to be replaced is how your vehicle reacts. In some cases, the car will struggle to maintain speed and a potent smell can be noticed if the parts are burnt out. Unfortunately, you will not be able to check the brakes because they will be so hot from the friction caused by braking. This process has to be done immediately to avoid having to replace the rotors as well. Dragging callipers will score and scratch the rotor surface causing it to become worn down and damaged. In this case you will either have to machine the part or replace it completely.
For the mechanic to reach the callipers, they will have to loosen the lug nuts and remove the wheel. If rubber line screws are attached, they will have to remove the rubber brake line at the far end first. Once the component is out, the brake pads will have to be removed. At this point, the mechanic is constructing the calliper to be a component on its own without tampering with the rest of the system. This includes the seals within the component and the piston that contains the hydraulic fluid.
In most cases, the reason why the calliper drags is due to serious corrosion that occurs over time. Grits and rough surfaces will have to be polished down for a smooth surface. Remember that the calliper has to be functioning at all times. Any roughness and corrosion will compromise the fluid from making this clamping motion.