Typically the "sniffer" test will determine if combustion gasses are present in the coolant which would indicate a faulty cylinder head gasket. If an intake manifold gasket leaks coolant internally, it will drain into the crankcase. Pressure test the cooling system, and watch for external leaks. If there are no external leaks and the pressure still drops, the intake gaskets would be highly suspect, (very common on 4.3L engines).
Provided the oil pressure is within specs, there is no internal damage to the engine, and the basic engine is in good condition, it may be a worthwhile investment to replace the intake gaskets.