Head gasket

If you are asking which side on a V8 the simple answer is to replace both sides because if one failed the other is likely close itself and you already have it torn down for one side. However, if you are trying to minimize the project there are a few ways to tell. One would be to do a leak down test, for this one you don't need the fancy one that gives you a percentage, just a spark plugs hole adapter and a source of compressed air. You then remove all of the spark plugs, during this step look at the plugs, usually one that is burning coolant is going to look very different than ones that are not. So, say you find 7 plugs that are tan and "dirty" and one that is clean but has a strange color, that is the cylinder that has a problem. Next you pick one cylinder to start with, now rotate the engine until that cylinder is on the compression stroke and at top dead center. If you install the plug adapter first this can be easier as the piston coming up on compression will push out air as it compresses it.
Now use a bar or one of the clamping methods on the flywheel to hold the engine at TDC and slowly apply air pressure, at the same time watch the coolant in the radiator or overflow, if the gasket is leaking you will normally see air bubbles.
https://www.2carpros.com/articles/how-to-test-engine-compression
This is sort of the reverse process of a leak down but if the gasket is bad enough it could show up with a compression test.

You can also use a pressure tester on the cooling system to verify that you have a leak
https://www.2carpros.com/articles/radiator-pressure-test
Then rotate the engine with the spark plugs out and see if you get water out of one or more cylinders.