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Well, it usually involves using the Steven’s pressure tester, which few have and is pricey. What type of outboard is this? Am assuming it is some sort of classic OMC, so will start with gear lube inspection. I am assuming there is water present which is why you are asking. The next step is to make sure the leak was not caused by a missing drain plug seal, or perhaps two seals on one screw and none of the other. Oftentimes these seals stay behind in the housing when the screw is removed, other times the seal comes out with the screw. It is hard to see the seals left behind in the housing, especially covered in gear lube.
So, assuming the drain plug seals are not to blame, we move on to the pressure test. Install the lower drain screw and seal, then screw the pressure tester into the top vent hole. Make sure the pressure tester has some sort of seal on its threaded adapter. It is best to use the top/vent hole for the pressure tester to keep residual gear lube/water from running back through the line and into the tester. The seals are tested using relatively low pressure, 5-7PSI. Over pressurizing the unit can over come the seals ability to seal, and might actually damage them. At this point it is helpful to submerge the gearcase in a bucket of clean water so leakage issues can be easily pinpointed. It is best to rotate the shafts, and move the shift rod up and down while testing as well. You can use a higher pressure setting to check orings and gaskets, 12-15PSI.
Just a side note when checking OMC 5.5-7.5, 10hp gearcases. Do not remove the water pump screws, then perform the pressure test. The water pump screws hold the upper bushing seal housing in place as well. Pressurizing the gearcase with no water pump screws will probably just push the bushing/seal housing up and off its gasket. I realize it is tough to pinpoint leaks up there with the water pump in place. You could remove the impeller housing/impeller, then secure the upper bushing seal housing in place with shorter 10-24 screws to hold it in place while testing.