What the ESD Association says about testing wrist straps:

Per ESD Handbook TR20.20 section 5.3.9.1  Wrist Strap Checkers - “Wrist strap checkers have historically been the primary tool for testing the operation of the wrist strap system including, the wristband, grounding cord and most importantly, the electrical integrity of the contact between the wristband and the wearer. Wrist strap checkers measure the resistance of the entire system.”