A compression check can tell a lot about the health of your outboard engine
A compression check can indicate that the piston rings are working properly and that the cylinders are in good condition. The test measures how much pressure is built up by the motion of the piston inside the cylinder, given in pounds per square inch (PSI)
A compression test is simple, but the many different types of outboards can add many complexities. Your engine may require different tests depending on factors such as whether it's a two- or four-stroke, has fuel injection, has computers onboard, is hand cranked or has a starter motor, and how its ignition can be disabled. Because of this, unless you are very mechanically inclined, let a trained professional check it.
If your test reveals low or erratic readings, you may need to check the engine cylinder head(s) to inspect the piston/cylinder condition. Again, I suggest you might want to defer to your mechanic, if you haven't already.
If your engine produces healthy, even readings, record this information for future reference as a baseline.