Pontiac Transport coolant leak

I have a 1998 Pontiac Transport which is leaking coolant internally into the engine. I just had the intake manifold gasket replaced, and now the same mechanic says it is a leak in the head gasket. How do I know if the problem truely is the head gasket and faulty installation of the intake manifold gasket?

Do a compression leakdown test. Each cylinder is pressurized and the rate of pressure loss (leakage) is checked. If it’s leaking, chances are the headgasket is bad. A Haynes manual should have the protocol described.