It actually is hard to describe what you feel when you work a point of sale touchscreen that needs to be re-calibrated as each of them has different errors.
Generally, you would look for some of these symptoms.
1) Does your touchscreen feel like it is responding too slowly?
2) Do you feel like your touches are not correctly being detected?
3) That the touch experience is no longer smooth as it was before.
4) That you need to frequently press a button a few times to work.
If you feel something like this is happening you might try re-calibrating your screen. Each screen is different so you will need to look at the manufacturers specifications.
If after doing this, it does not fix the problem it probably needs a repairman? At least the repairman will not charge you for the time taken to do the re-calibrating.