How do i tell if my outboard is a 2 or 4 stroke?

it would be a rare 15 hp engine in 1999 that was 4 stroke....two ways to tell...there will be an oil reservoir somewhere under the engine cover for you to put oil in,,,,,,,,,,,,look for a small plastic container marked "oil"......the other way to tell is a 4 stroke is WAY heavier than a 2....a 15 Johnson 2 stroke should be about 50-60 pounds.....a four stroke 70-90