If you pull the cap off the distributor you can watch the rotor turn around when the balancer moves. If you rotate the balancer one direction and then back it up and the rotor takes a long time to catch up then the chain has excess slack in it. Be careful not to accidentally loosen the balancer bolt turning it back and forth.

If the rubber on the balancer is dry rotted and hard as a rock it may be time for a new one even if it hasn't started to slip yet.