One way you can try to tell if the existing coating is paint or gelcoat is to look at a scratch in the surface, in an inconspicuous location. If the surface is scratched, gelcoat will be the same color throughout; paint will show primer or existing paint beneath the surface, and the mil thickness of paint is much thinner than gelcoat. But this isn't always a good way to tell, depending on the thickness of the coating.
People usually ask this question when they decide to paint their boat and want to know if the existing coating is paint or gelcoat. Why does this matter? You can apply gelcoat over existing gelcoat without priming first. If you want to paint over gelcoat, some paints may require you to apply a primer first. If you want to apply gelcoat over paint, you also have to prime first, or if there is gelcoat under the paint, remove any coatings to expose the old gelcoat before applying new gelcoat. The bottom line is, when in doubt, prepare the surface and apply an appropriate primer for the coating you want to use.