A simple leak down would be to put a compression tester on the cylinder and crank the motor a few times, once you get your reading let it sit for a few min at pressure. Say it's at 160psi it should stay relatively close to that but off after a couple min it drops to 120psi there is an issue.
A leak down kit is pretty much the same as a compression tester however instead of cranking the motor it allows you to use a compressor to pump air into the cylinder which not only will show on the gauge but in some cases lead to bubbles in the reservoir.
I am no tech by any means but that's what I gathered when troubleshooting my motor...