What are some signs that a head gasket has been

YES IT IS. ALSO ENGINE COULD RUN ROUGH, AND A NEED TO KEEP ADDING WATER. HAVE A BLOCK TEST DONE, IF WITH BLOCK TEST TURN YELLOW EXHAUST FUMES IN COOLING SYSTEM 99% OF THE TIME BLOWN HEAD GASKET.

yes bubbles in the coolant tank, white sweet smelling exhaust smoke and engine overheats or missfires

Yes, those are all good ways. If you start the engine cold, with the radiator cap off, and it pushes water out, thats bad. To be absoletly sure, have a shop check it, they can use a "block checker" to detect CO in the cooling system. If you do take the head off, have a machine shop check it for cracks and they may need to straighten it.

Yes..in a way..you will see smoke trying to find its way out and sometimes you can see oil in it in the worst cases..Also start engine and rev at high rpm and watch the exhaust if there is any water coming out.
Now there are cases when the damage is very minimal and this is where you have to be careful then there is a special instrument that fits on top of the water container(for the radiator) which then gives you a color change if there is any damage to the gasket, of course the test needs to be done when you have a running engine.