Given your answer to Greg's question I would not twist the two black wires together. Ideally, if you have access to the wiring for the fan, take some detailed pictures. Sounds to me like the fan is wired with two hot wires. If you can't get to the fan: (Goes without saying, turn the power off at the circuit panel when you do this.) Disconnect the two black wires from the top of the combo and put wire nuts on the exposed ends. Turn the power back on and see if the fan works with the other switch. If the fan works then you are good to go. At least 1 if not both of the black wires that are disconnected from the combo are likely live. If you have an electrical tester you can determine if one or both is live. Keep them capped and put them back in the box when you install the GFCI outlet.

okay so your basic switch layout is going to involve two black and two white wires. you join the white wires and the black (hot) wires attach to the switch. what the switch does is break the flow of current going through the hot wires. i am a little troubled by terms like "...power the switches" because switches aren't really powered, they are what break current. i am unclear on where you are getting these wires from. please provide a new description of what you're working with. this stuff isn't hard, but it does take some background to know what does what and also appreciation that you could kill yourself.

The little outlet testers can't test GFCI without a ground. They leak a small amount of current from hot to ground which won't work without ground. As already suggested, the easiest and most effective way to test is use the TEST button on the GFCI. Randy: Actually to this day bathroom receptacles are allowed to be on circuits that serve only receptacles in one or more bathrooms. The other option is that a circuit may serve only outlets (receptacles and lighting, etc...) in a single bathroom. As Mike points out, adding GFCI's is cheap and easy if they are found to not be present.

"Only the breaker would, if enough total current was passing." Well, sort of. Just to dispel any possible confusion: A GFI breaker implements two functions. The first is overcurrent protection. The breaker trips if the total current drawn from the circuit exceeds the breaker's rating. For branch circuits this is normally 15 or 20 amps. The more it exceeds the breaker's rating, the faster the breaker trips. The second is ground fault protection. The breaker trips if it detects that the current is asymmetrical across the power line. The amount of asymmetrical (ground fault) current that trips the GFI part of the breaker is MUCH lower than the current that trips the overcurrent part . (Confused yet?) To explain this second function: it's not 100% accurate to describe it this way, but it's easier (I hope) to understand if you think of the electricity as going out the hot side of the receptacle and coming back in the neutral side. The GFI monitors both of these. If the amount of current coming back is less than the amount going out, some of it must be "leaking" to ground. That leakage might be going through a person or animal, risking shock or electrocution. So when the GFI sees that, it instantly opens, shutting off the power. With most GFIs, an imbalance (fault current) of 5 milliamps (0.005 amps) is enough to cause a trip.