Dan,
Do you have a leakdown tester? If so, you can see if the head gaskets are sealing correctly. Another way to check for this is to use a chemical indicator fluid that changes color if combustion gases are entering the cooling system. A company called Lisle Tools makes an inexpensive combustion leak detector kit (P/N 75500).
Even if either of these tests aren't positive, at least you've ruled them out as possibilites.