What a question! Despite it being simple, there is no simple answer to that one. Handling badges, studying badges, and lots and lots of research are needed. Even today, most experts can be fooled by some repros. There is no simple method. A case by case approach is probably the best.
In my experience you look for poor detail on the front and rear of the badge, this usually means a copy, and extra flashing and misaligned features. It is a steep learning curve a many new collectors see a badge and want to immediately grab it, take your time and look at it carefully. Also if the solder on the lugs look new, its a copy.
Others may have hints as well.
Here is a copy of a badge and what to look to look for in this particular badge.
Yes, there were kc badges made post war. Not restruck, but continued to contract and strike them from the existing dies which were held in most part by NDHQ. Most collectors don't realize that the qc issue of militia badges didn't really get into swing until about 1958, and for some units the issue was much later. For example, the Elgins didn't get a qc badge until the early 70's.