From the Trademe site, a reseller points out the salient differences between the real SM58 and the typical fakes that abound.

Be aware that there have recently been quite a few fake Shure SM58's on trademe, if you buy one of these you may as well just go down to Dick Smith and buy a $20 mic as that's all it's worth.

"If you have bought a SM58 on Trademe and are concerned that it's a fake then have a look at the pictures on this auction. In all these pictures the one on the left is genuine, the one on the right is fake.

If yours is not identical to the one on the left in all aspects; then  you've got a fake. Take a look at the metal ring under the grille, if it is flat then it's fake. If it is contoured like the one on the left it could be real but it's still not guaranteed. And yes they even copy the warranty card too although this is different to the genuine.
ne.
SHURE SM58 MICROPHONE
REAL VS. FAKE
Radio Brandy Micro Broadcasters Homepage
IS AN INDEPENDENT PROJECT BY STUDENTS OF THE RADIO BRANDY BROADCAST WORKSHOP
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