A picture would help. Otherwise, look for markings, like manufaturer names/logos near the cockpit (sometimes at the aft end, near the rubrail, sometimes near the sides of the cabintop) or on the top of the mainsail.
On the top starboard side of the transom should be a series of letters and numbers. This is the HIN (hull id number). Every boat built after the early 70's has one, though they are us times obscured. If you can find it, there is a USCG database you can type it into and it will tell you, or report it here.
If you can't find it, then post some pics and we might be able to help.