Identifying SS guns or weapons is a tricky issue for several reasons. First, the SS, especially the Waffen SS, did not generally issue their own specific firearms. Additionally, there are a number of ways collectors might be fooled into thinking they have obtained an authentic piece when that is not the case. If you do your homework and can establish provenance, however, you could find the collector's item you so desire.
Note that the firearms issued to the Waffen SS were the same weapons that were used to supply the regular German army forces. Because the SS was an elite corps, it frequently received the most recent weapons before they were issued to the regular army.
Identify the common models. As with the regular corps, the typical rifle of the SS was the Mauser Karabiner 98k. This is a bolt action rifle holding 5 rounds at a time. The common sub-machine gun or machine pistol was the MP40. This is usually identified by the absence of the wooden butt stock. In cases where the stock was added, the model is the MP38.
Observe the holster. If you have a German pistol from that era, it is probably the now-famous Luger. This was the sidearm used by the SS and regular officers. There may be no mark identifying the Luger as SS but there may be one on the holster, if you have it. The marking may be the death's head insignia.
Procure a reference guide for weapons. A good weapons resource will provide you with additional information and images you can use in your identification. The Osprey Men-at-Arms series is an excellent example and includes a volume specific to the Waffen SS.