If its still acting up, have it tested or get yourself a combustible gas tester as mentioned before.
If its a head gasket, doing a simple compression test should answer that
I'll agrees with the first two suggestions, but not the third. if it's leaking bad enough to show up on a compression test there wouldn't be a question or need for the first two as it would be pretty obvious due to loss of coolant, overheating, missing etc.
I disagree, there's head gaskets that catastrophically fail and ones that fail but only act up when full operating temp is reached, repeating this many times when brought to temp but not bucking/carrying on for quite a while, then it $hits the bed all together. Heres another kinda easy test, put a borescope down the plug tube and tell us what the piston tops look like.
Keeping in mind now, 1 of your 3 criteria is already met.
