Instructions
Examine the shape of the helmet. A Stahlhelm should have a distinctive flared rim, reminiscent of an old-fashioned coal-scuttle. The original M16 model (in other words, the model first introduced in 1916) had steel lugs on each side, and these persisted on subsequent models until the introduction of the M35. The rim on the M35 was more streamlined, with a rolled edge.
Run your fingers around the edge of the visor. As the German war machine struggled to keep up with demand, the impeccably well-made M35 was "dumbed down" into the M42 and M44, both of which have straight rather than rolled edges.
Inspect the lining. The M35 had a very practical leather liner held together at the crown with a drawstring. It should be stamped with the helmet size, the manufacturer's name and date of production, or, for helmets made after 1942, with a serial number. There should also be another stamp on the chinstrap.
Look closely at the insignia, the markings on the sides of the helmet. A helmet dating from World War I is unlikely to have any, but they were widely worn during the Nazi era, not just by the military but also by civilian groups involved in home defense. Perhaps the most famous is the double lightning-bolt of the Waffen-SS (more properly called a "silver runic shield"), but for other insignia check out the Online Guide to World War II German Helmets. These insignia were produced by printers in the form of decals. Once applied to the helmets, they were covered with a durable lacquer.