Previous posts are correct about brushes. They're cheap (~$8/set of 4) but the idea is to change them before they fail. Do not run your generator if you are not sure the brushes are ok. If they stick or are worn out, they arc and burn the copper commutator segments.
You can test the generator by forcing it to produce full voltage (around 15-volts). Take the field wire off your regulator and connect it to BATT. The ARM voltage will increase to full volts. You can run this for up to two minutes, which should be plenty of time to troubleshoot. Either keep your lights off, during this test, or don't rev the engine real fast. Your meter should show when you approach 13.5-volts (nominal charging voltage).
Turn the headlights on for a while to drain the battery. If the regulator isn't sending armature current to a battery that is partially drained, you will know.
Your GEN light only comes on when the battery voltage is higher than your generator is putting out (like when you first turn the key on before starting your engine, or if you throw a belt).
All the current to charge your battery goes through the generator brushes. They should last for about 50,000 miles.
By contrast, an alternator has two brushes, but they only pass about one amp (to excite the field). Those brushes are much smaller and should last 100,000 miles. The whole brush holder w/brushes cost about $15. On Ford alternators, they are accessible from the back without removing the end-plate.
Hope this helps. - Dave