I see a lot of WWII patches offered that are not from WWII. For example, I see good 101st Airborne patches and a greater number of copies, fakes, and mislabelled ones sold without clarifying-or worse claiming to be WWII vintage.

This is a lengthy subject-I'll give some basics here.

In WWII, US Army shoulder patches -SSI, aka shoulder sleeve insignia- can and often do have a variety of manufacturers. I have handled hundreds in 30 years of collecting from actual vets. And for years I have learned that many people will  just assume a design used in WWII was made then--particularly the scarce insignia. The most common things to look for are easy to spot in eBay listings when you can look at the front and backs of a patch. Those sellers who do this appear to be the most knowledgeable, letting the buyer see the manufacture style. The patch sells itself.

Beware sellers who have hidden the bidder identities and feedback comments. They may be hiding something.

Most WWII shoulder patches have been embroidered in a silk/rayon weave on canvas, then cut close to the edge. You will see the tan or olive edging. Known as a 'Cut Edge.' See the  fraying or tan cloth below on a 30th Div. patch

This appears to be WWII vintage. On the back one would find olive or white cotton thread. If these are post war replicas, on the back the thread would likely be a thin nylon, sort of clear, or a synthetic blend. Newer fakes will glow under a black light. I used to find a small stray thread on the back and burn it on my electric oven coil. If it is synthetic, it melts. Cotton burns.

In the 1960's virtually all patches acquired a 'merrowed' edging, which was done after a patch was cut. An edge is embroidered to make a nicer finish. This finish, however, is a bit bulky and takes the detailed edges off a patch such as the 101st Airborne shown: These would generally have synthetic back thread as well.

  Note this is not a WWII patch on the left.     The patch to the right is.

Many of what I consider to be true fakes have an outright wide variety of design from somebody not exactly getting the right detail. Many of these are very thin and flat with cut edge, or sort of puffy. The majority of authentic WWII 101st patches you find should remain reasonably close to the detail shown in the patch on the right, whether the tab is attached or not. I've seen more new variations to this basic design in 5 years than the 40 years after WWII.

For some great details and insights, check  out Mark Brando's website on the 101st, with numerous examples.