Take your radiator cap off while the car is cool, start it and let it warm up without the cap. If the coolant level is low, or drops at all during this cycle, top it off. If it spills out, so what. If it spills out and it's being pushed out by escaping exaust, it's a head gasket. Usually, you can see exhaust gas escaping. Like '93TurboD said, you can use a gas analyzer (any shop with a dyno will have one), but it's usually visible. Coolant shouldn't continue to spill out of the car until it boils. Exhaust gasses should be escaping even when it's cold (if this is your problem).

Pressure testing your radiator can also help diagnose your head gasket.

Oil floating in the coolant overflow bottle is a really bad sign as well.