As ygmn stated above, you are looking for a cylinder or two that shows a much lower pressure than the majority of the rest.
I would also like to say that you should have all of the spark plugs out while doing your testing.
Start at one corner and work your way around, making notes on your pad as you go.
You COULD do the testing on a warm engine, but I have always done so on a cold engine.
No need to make the job any more uncomfortable than it already is.
You will also see references to both a "wet" and a "dry" compression test.
The dry test has always told me what I needed to know, so I don't bother doing a wet test.
A wet test involves squirting a SMALL amount of motor oil into the spark plug hole in order to possibly seal up some leaky rings temporarily in that cylinder and see if the pressure goes up on the next test.
Too much trouble, in my opinion, for most cases.
Also, note that when you are doing your test, only spin the engine five or six times per test.
You will get your reading in about that amount of time.
I had a 2006 CTS that was having quite a few issues, one of which ended up being a dead lifter, that required quite a few repeated compression tests to be ran.
Since the starter was most likely the original in that car, we ended up killing the thing and that was not an expense that I had been expecting during that repair session.
