I considered having a stamp made up with Firma Quatsch und Blödsinn, but markings can be modified, eradicated or even used in claiming an item is an original (Q & B...oh, yeah, they made helmets for the Kaiser...)
Modern materials and construction techniques make it impossible to pass these replicas off as originals if the person looking at them has read a good reference book, studied originals, and/or learned from collector forums.
If the big bucks shelled out on the auction sites for obviously bogus items is any indication, some collectors are simply victims of their own willing (and obstinate) suspension of disbelief:
http://www.icollector.com/Militaria-helmet-w-horse-hair-plume-wartime-Prussian-E-M-Tschapka-Ulanen-Regt-5-6-15-or-16-h_i9445823" onclick="window.open(this.href);return false;
http://www.icollector.com/Militaria-helmet-w-horse-hair-plume-wartime-Prussian-E-M-Tschapka-1st-Garde-Ulanen-Regt-Exc-ori_i9445829" onclick="window.open(this.href);return false;
The filz Tschapka on eBay is okay for a display item, but it's nowhere close to "right." Polyester thread, no stitches on the mortarboard top, etc. Put it under UV light and it'll glow like a Christmas tree.