It's done with a tool made for that purpose. You turn the engine by hand until one cylinder is near top dead center and both valves are closed. Then apply air pressure through the spark plug hole with a compressor and the leaktest tool. If there are any leaks you will hear air escaping and the gauge will tell you how bad it is. I suspect that the exhaust valve on your non-running cylinder is leaking, and if it is you will hear air hissing in the exhaust pipe.
This is an example of the tool -
https://www.ebay.com/p/8018342597?i...C6eHf84IocQqTxkixc9qyTwZHEhVkqThoCPQEQAvD_BwE
Any automotive parts store should have them, perhaps for rent.