Neumann does not put a brand name on their capsules so it takes some experience to spot the genuine article. To make matters even more confusing, now days you have all of the reskinned Neumann capsules being sold, so the question is not only how can you spot a genuine Neumann capsule but how can you spot a reskin? Neumann K67/87 capsules have also changed over the years. The first thing to look for is to try and spot the era the capsule is from. The earliest one had a metal ring around the diaphram, then Neumann used a sort of fiberboard like material for the ring, then they went to brown plastic and then finally to a white plastic ring. Also, if the mic was sent back to Neumann for repair over the years, they would often replace the capsule. So you can have a U87 from the late 60's that used to have a metal ring capsule but was sent it for repair in the 80's and came back with a white plastic ring capsule. I don't think any one era of Neumann capsules is better or worse than another but there can be some slight differences in sound over the years.
All genuine Neumann capsules that I have seen have hand written numbers and codes on the diaphram rings. The 2nd digit on one of the codes is for the year the capsule was made, but you have to know what decade the capsule is from since the first number in this code is not for the decade. I am not sure what the other codes are for. It's very hard to spot a reskin but the color of the gold on the diaphram is slightly different and some experts like Klaus Heyne can spot the reskin from an original diaphram. If you could post a photo of the capsule, someone here will be able to tell you if its a genuine Neumann capsule, a reskin, or some copy not made by Neumann.