1977 aspen 225/6. When at idle it vibrates quite a bit. Does not idle rough but resonates/vibrates. how do I tell if the harmonic balancer is bad? Can they go bad?

Check torque converter bolts to make sure they aren't loose.

Momentarily disconnect the belts, 1 at a time, start the engine and see if it goes away. Could be bad alt. or WP bearings. Just had a bad WP a month ago on a friend's 78 Volare wagon. Solid O.E. fan w/spacer, fan was missing 1-1/2 of its 5 blades. (Ended up costing me a trip to the ER and 9 stitches on the back of my left hand from the busted, and sharp fan!)