Many online collections buy and sell World War II helmets from various countries and units. German War Helmet, Snyder's Treasures, Helmets of War, SS Steel Inc. and WW2 German Steel all have examples of genuine World War II helmets. Helmets from this era were manufactured between 1916 and 1945.
German helmets are made of steel and are most valuable when they are unmodified, according to German War Helmet. The definitive book on these helmets is "The History of the German Steel Helmet 1916-1945" by Ludwig Baer, a 448-page volume that contains pictures of every variety of German helmet made during World War I and II.
Authentic helmets show wear-and-tear on the exterior and interior including decals, paint and liners. Liners should fit inside helmets properly. Sometimes, soldiers would switch helmet liners later in the war. Helmet liners fit with just a small gap between the liner and the inside of the helmet.
Camouflage helmets have been known to present problems for collectors, according to German War Helmet. Camouflage colors were sometimes painted on after the helmet was removed from action. Rust can come through the paint job to reveal newer paint on top of an original helmet. Rust shows up as darker colorations underneath newer paint.