As the distance between the separated pads and the attached discs is only a small distance, the unit of caliper travel is usually only a few millimetres. Theoretically, if the wheel and disc are not crooked, then the gap between the pads and the disc will not exceed 0.1mm, so the piston travel of a floating caliper will start braking as soon as 0.2mm is moved. Conversely, if the piston returns to the caliper within 0.2mm of travel after the hand releases the brake lever, the pads should leave the disc.
However, in real life this does not always change as described above. Often it is necessary to look at whether the pistons are not acting smoothly due to dirt, or whether the pads have not left the disc. Ideally, the brakes should be applied within a very short piston travel, but for the pistons to stay in place at all times, the pistons must be able to change in response to the rider's actions, even if the caliper seals are only a small roll back distance. To achieve this, it is important that the surface of the caliper is smooth and that it is well oiled and lubricated during regular maintenance.
If the surface of the piston accumulates a lot of dust or dirt from the pads, the oil on the piston and the caliper oil seal will deteriorate or dry out, and the piston will not return to its original position. Overly sensitive brakes are easy to detect, but if the pads are attached to the discs at all times and make no sound, it is generally difficult to detect.
For example, if you let the tyres come off the ground and the engine idle, and the idling stops immediately, you can press the caliper and force the brake pads away from the disc before trying to idle the engine. If there is less friction and the idle time is longer, then it is likely that the brake pads have been pressing against the disc. To be on the safe side, try the brakes again and let it idle again. If the tyres feel chunky, then you can be sure that it is because the lining has been pressed against the disc.