You can rent cooling system pressure testers.
They connect on rad cap opening and then you can pump up cold engine cooling system to 16psi pressure, normal hot engine pressure(when it leaks most) is 16psi.
Doing this on a cold engine allows you to pin point the leak easier since you can feel around cold engine parts.
You can also get dyes that are added to coolant, then run the engine and shut it off after it warms up, you need a UV(ultra-violet) light to see the dye on the outside of the engine.
Water pumps have a hole on the underside that will leak when the shaft seal starts to fail, that should be easy to ID on cold engine.