It a very valuable tool, one I've been using a USB microscope to examine SS and other helmet decals for years now and there are many pinned threads on the topic over on GHW. In fact the USB scope is becoming de rigeur in helmet collecting, a must have.
I've been meaning to post the same here but many helmet guys are members of both fora so I haven't spent the time to do it here. Below are some links. At 20-400x the fakes simply break down but you need to examine hundreds of real - and fake - to kow the "fingerprints" of both and what to look for. I also have a background in graphic design and printing so applying that knowledge to decals as well certainly doesn't hurt.
One thing I would caution is knowledge doesn't come out of the box. A fake under magnification can look real if you don't know what real is.