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.”

ANSI/ESD S20.20 Paragraph 6.2.2.1 Personnel Grounding Requirement "All personnel shall be bonded or electrically connected to ground or contrived ground when handling ESD sensitive items. When personnel are seated at ESD protective workstations, they shall be connected to the common point via a wrist strap system."