Leak-off testing/Compression Testing??????

I apologize for this nub question, but it has to do with leak off testing and compression testing. I have read somewhere hear that to test for blow by you should perform a leak off test. I understand leak off testing just doesn’t test the cylinders for blow by but valves and other stuff. So….how do you know which is the problem. Also, would a compression test be better suited for testing whether or not you have blow by in the cylinders???????

Thanks in advance for the help.