An old mechanic told me this. get the engine good and hot, release the pressure in the cooling system then take it for a short drive. There should be much less pressure build up since the coolant has expanded to the maximum.
If your getting a significant pressure increase within a couple miles it may be the HG.

I have not actually tried this but it does make sense

I also use the "squeeze test" on the top radiator hose. First run it should feel very tight, second run, much easier to squeeze.

I just had a radiator pop from what was supposed to be a new 75KPA cap that didn't vent, my top hose was rock hard!