Its pretty annoying actually. There are 3 big huge give aways as to wether or not the head gasket has gone, but if they are not obvious it can be harder to determine the problem.
1) Bubbles in the radiator expansion tank
2) Oily/Creamy substance in the radiator or on the dipstick. (creamy substance under the filler cap isn't always a giveaway as moisture collects there on most vehicles and will emulsify with oil over time)
3) Uneven running, getting too hot very quickly, dissapearing water levels.
If the thermostat has gone, then it should run perfectly fine with no issue, but it will always try and get into the 'red' on the temperature gauge. The issue is purely temperature related.
However. If the car has gotten very hot i.e - past the red, off the gauge. Then the head can warp by the tiniest amount, causing a head gasket failure.
If you need a hand or want some options or it looking at, send me a PM.