Coolent hose leak. probably heater core

[cre]

Sadly, it is probably one of the two hoses which connect behind the head... a major pain to replace.

The pressure tester is very simple... a radiator cap (which doesn't vent) and a pump. When the engine is cool, put the cap on, attach the hose to the pump and start pumping away. It's easiest to do with two people, one pumping and the other looking around with a towel. Use the towel to dry the wet area as you close in on the leak if it's not something you see right away. Also, make sure the coolant system is as full as you can get it... it doesn't work if the leak is above the coolant level.