How to find which side (rear or front) head gasket is blown

We always replace both head gaskets. If one is leaking due to corrosion, mainly from old coolant that has become acidic, it is just a matter of time before the second one starts leaking too. Your customer is not going to be pleased with you for trying to save a few bucks now, then having to do the job over a second time later. That is like buying one new sock to replace the old one with a hole in it.

If the leak does not seal itself when the engine is off, you can find the leaking cylinder with a "cylinder leakage test". That involves forcing compressed air into the cylinder, through a tester that regulates that air pressure to around 30 psi. The piston must be at TDC on the compression stroke. The gauge on the tester will show the percent leakage, and you can listen in four places to identify the cause of excessive leakage. With a leaking head gasket, you will see bubbles in the radiator. For a leaking intake valve, you will hear hissing at the throttle body. For a leaking exhaust valve, you will hear hissing at the tail pipe. If the piston rings aren't sealing well, you'll hear hissing at the oil cap or dipstick tube.

Yes, I included the comment about the leak not sealing up when the engine is not running. When the leak is small, it might take the high pressure of combustion gases to force those gases into the cooling system. The tester limits air pressure to around thirty pounds, and that might not be enough yet to cause leakage.

Yup. That is the only proper repair. If you are losing coolant and you want to verify it is due to a head gasket, add a small bottle of dark purple dye in the radiator, then check a day later with a black light. The dye will show up as a bright yellow stain. If a head gasket is leaking, you will find the dye inside the tail pipe. Auto parts stores will have the right dye for the fluid being tested, and those that rent or borrow tools should have a black light.