A friend of mine recently accidentally bought a fake Shure SM58 on eBay.   It was a very convincing fake, but after holding it in my hand for a couple of minutes, it was apparent.

Firstly it is a microphone, it does work, and he got it cheap, but still it's not a Shure.  

The Box is VERY convincing.    The instructions/technical data sheet was photo copies, and a touch off registration.    At first the mic felt about right, size and weight, however the biggest give-away was that the microphone was branded a SM58S.   There is of course such a mic, but the S is for Switched, as in it has an on off switch on  the body of the mic, and this mic did not have one.   Once I realized that, it stopped looking so convincing, little details, after a couple minutes it was obvious.  

So BE careful when buying a SM58, especially from a non dealer.   Look closely for the quality details, and in this case the forge had the "S" but not switch.    Likewise if a microphone has a switch and no "S" be suspicious.

The older US made SM58's seem the be the best anyway, so consider buying a used but not abused 80's model.    In the 1990's they began Mexican production, and the microphones change ever so slightly.   They are still fine mics, and official Shure products, but they feel a little lighter in the hand, and have a very slightly different sound, depending on who you ask.