My 2 cents worth
If the engine is apart I change them with new gears and a double roller chain. If it is a SBC with the nylon teeth change it. To check with out pulling the engine apart I pull the Dist cap and watch how much the rotor moves when turning the crankshaft back and forth with a socket and breaker bar on the big pulley nut, . My guess is if it has more than 5 * i change it.