IANAL nor expert but this is what I use as guidance:
If it's chambered in 5.56/.223 or .223 wylde and is a direct impingement system (I.e. swappable parts with your standard AR) it must have a heavy barrel. Not marked on the barrel but sold as such. I have an 20inch HBAR RRA upper and I keep a copy of the description in the buttstock just in case.
I think piston 5.56's are good to go regardless of HBAR. I know that any other caliber thats DI should be good to go (I.e. 300BO).
If it's a pistol w/brace then HBAR is not needed with any caliber
Of course the usual NFA laws apply, but these are to my knowledge the requirements for any maryland AR style rifle. It's way more complicated than this (I.e. features) but this is a good starting point.