I doubt that any firearms markings or currency authentication expert would need much time to study the stamping on the left, above, before reaching his conclusion. Too many things jump right out at you. LugerVern mentions a couple.
Having the genuine article for comparison usually makes it easy, except when the forger is a real pro. If there are small but distinctive irregularities in the original, they should also appear on the marking in question. If there was symmetry and perfection in certain features of the original, the same should be true of the marking under study.
Take a look at the lack of parallel alignment of vertical stripes on the shield in the left photo. Compare it with the photo on the right. Anybody need more time to deliberate?
People who represent rare and valuable things, on the Internet, as being genuine should assume that not all their viewers are yard sale browsers who will only take a quick look from three feet away.
With no bona fide exemplar for comparison, and quickly viewing it from a distance, the effort on the left is not all that bad.
