Usually one can deduce a pen's materials from its date, which can be deduced from its design. Pens predating the mid-'20s are almost always going to be made of hard rubber, though a handful of early models incorporated casein, transparent Bakelite, or celluloid. Multicolored pens with pearlescence or translucence are almost always going to be celluloid, as are just about all pens of streamlined (as opposed to strictly cylindrical) silhouette. Parker 51s are made of acrylic resin (also known as Lucite or Perspex), but in the postwar era most penmakers went from celluloid to cellulose acetate and various injection-moldable polymers.
Hard rubber can usually be recognized by its distinctive sulphurous smell, especially if first rubbed with a finger. Celluloid also gives off a distinctive smell: when its surface is wetted, it will have an odor of camphor one of its key ingredients. In some cases, however, more scientific methods are required. One can test for celluloid by taking a tiny, tissue-thin scraping from a hidden location (typically the inside of the cap or barrel), which is then placed on a glass slide and watched under a microscope as it is exposed to a droplet of acetone. If celluloid, it will promptly dissolve; casein, Bakelite, and acrylic will be unaffected.