@EiPersonal,
If you can't get the wheels off for measurement, check the ridge between the area in contact with the pads and the part that looks new. You're looking for a 1 mm ridge per side, and you should be able to make a pretty good guess. While you're at it, look for really deep grooves.
At 20,000 miles they should be good. If they are not, possibly the driver has been abusive in other areas as well.