Pump cold system to 16 psi and watch the gauge. Holds for 15 minutes = no leak. Drops fast = external leak. Drops slowly = internal (head gasket or cracked head).
The cooling pressure test isolates head/head gasket issues from external leaks. Equip yourself with a Stant ST-255 or rented Lisle cooling pressure tester. Cold engine: install the adapter on the radiator filler in place of the cap, pump to 16 psi (the rated pressure of the OE cap), and observe. A healthy system holds 16 psi for 15+ minutes with no drop. Fast drop with visible leak: bad hose, water pump, radiator, or freeze plug - obvious and external. Slow drop with no visible leak: internal leak. Now pull plugs and crank the engine over with the tester still attached - any pressure spike with cranking points to a cylinder leaking into the cooling system (cracked head, cylinder #3/#4 area is most common). Bubbles in the radiator or coolant pushing into the reservoir under load are confirmation. This test is the diagnostic that distinguishes 0331 head crack from other coolant loss.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.