For your question: "what are the symptoms of a generator gone bad", maybe try this.
1. Remove generator & with battery cables, touch the ground cable on the generator body while at the same time, touch the generator output connection with the other cable.
2. If the generator spins like a starter, it is good; if not it is bad.
3. If bad, either bring it to generator shop for test & rebuild; or, try to rebuild it yourself -- the first generator re-build takes longer; but the second one is a snap if you do not have too many senior moments.
4. If generator has bearings on both ends they are usually still good; however, if they have bushing, these bronze bushings can wear & should be replaced if worn because the armature can get off center & possible hit the sides & ground out.
Not a big deal to change generator bushings.
5. The more probable cause of generator failure is worn brushes & worn commutator. Worn brushes can cause the steel brush holders to move downwards & ground out on the commutator, and worn brushes can also cause the open grooves on the commutator to fill up with debris & ground out on each other in lieu of being insulated from one another.
6. Not a big deal to remove & provide new brushes. For cleaning comutator grooves, take a hacksaw blade & grind the sides on a grindig wheel so the teeth are narrow & clean out all commutator grooves to about 1/32" deep.
7. Prior to reassembly, get a test lamp or multimeter to insure continuity between each individual segment of the small diameter commutator to each individual segment of the larger diameter armature -- if no continuity, (sometimes rare), you need a new armature.
8. If you have any doubts about remembering re-assembly, use your cell phone to take picutes of disassembly so you can readssemble it to bring it to a repair shop & not be embarrassed to let the repairman know you could not fix it.
Hope this helps -- nothing like a good challenge !