|In order to make it a true 15hp. the inner exhaust tube must be changed out to the megaphone used on the 15hp. The carb provides most of the bang for the buck.
the numbers will tell you. also the 15 throat will be bigger than the 9.9 but I have no idea what the diameters. do you have a reason for thinking it may have a 15 carb on it?
It's difficult to be sure of anything without removing the carb. If you take a good clear picture through the throttle bore, with the throttle open, from the throttle end, then you can tell for sure what it is.
Here are some comparison pictures. The last picture shows a small machined shoulder on the rear of the venturi. This is a 15hp carb. The 9.9hp carb has the as-cast finish on the back side of the venturi.