The only way you are going to be able to tell what is going on is to yank the heads. Some people may prefer to pull the one side that is leaking and just replace one gasket, -I would not recommend doing that, -I'd do both.
As far as snake-oil sealing products go, -don't waste your time or money.
There is one exception to that though, and that would be the coolant tabs at the chevy store. they do work for small pesky leaks, but nothing as major as a head gasket.
It could be a gasket, it could be a cracked head, or worse, a cracked block. It is most likely a gasket, but the only way to find out is to pull the heads (they gotta come off anyway).