They put compressed air down each cylinder when it is on TDC (both valves closed) and you will get a reading of pressure lost. Then you will be able to hear/see it come from various places, Coolant, Exhaust, Engine Oil Cap and across other cylinders.
It is a much better way of checking engine health over a compression test.
Most decent workshops will be able to do it, just make sure they have a guage, some don't so you will not know for sure how much your loss is.
Hope that helps
-Josh