Instructions
Examine the frame and stock. Different models of the Mossberg have different frames. Some have a thumbhole in the stock, others are black synthetic, while others are wood. The Ruger and Marlin rifles are also available in a variety of stocks. Check on the stock and near the trigger guard for a company logo.
Look at the safety. The position of the safety might indicate whether your gun is a Mossberg. The Marlin rifle has a button safety located distinctly behind the trigger guard. Both the Mossberg and the Ruger have more traditional safety buttons located forward on the frame from the guard.
Examine the magazine. The Mossberg's standard magazine is a single-stack, metal 10-round magazine which extends from the bottom of the rifle down below the trigger guard. This is the easiest way to identify the Mossberg. The Ruger's factory magazine inserts entirely into the frame and is not visible from a side-view. The Marlin has a tubular magazine which extends lengthwise under the barrel.