Join Date: Feb 2017
Location: St. Louis, Missouri
Hinow, it's best to pinpoint coolant leaks before you tear anything apart.
I have a couple of my own from work, but you can usually borrow/rent a Cooling System Pressure Tester at most any auto parts store.
The engine can be cold. You remove the radiator cap, install the pressure hose adapter in its place, pump the tester until its gauge reads 10-13 psi....and watch all junctions, hoses, and the radiator itself for leaks. Unless you see a leak right off the bat, let the cooling system sit under pressure for 10 minutes, pump the tester to maintain the 10-13 psi.
The tester will have a radiator cap testing adapter so you can easily test the cap by itself as well.
If your buddy smells coolant from your exhaust, you can remove the spark plugs, pump the tester to 10-13 psi and let it sit under pressure for a good hour. Rotate the engine and look for coolant to be ejected from the spark plug threads. Look at the spark plug electrodes for a greenish deposit. I say this because, even though you replaced the head gasket, you can still have an coolant leaking under pressure into the cooling system. A leakdown test would be conclusive, but that would require compressed air and a spark plug thread adapter.
A cooling system pressure tester is a great tool to have for just checking cooling system integrity.