i found this on another forum. Maybe you'll find it useful
"I think it's pretty simple to do but here are the ways you do it:
1. If it's to good to be true it probably is. This mic is usually $99 USD new so if someone is offering a new 57 for $40 more than likely a fake. Even used 57s fetch anywhere from $50-70.
2. Look at the XLR connector at the bottom, on a real sure the pins will be labled 1, 2, and 3. Fakes usually have some other brand marking or no marking at all.
3. Look at the capsule head (58 only) if there is a caution sticker... fake.
4. Unscrew the windscreen (again 58) look at the bottom of the windscreen. There will be a lip here where the threads are. If the lip is flat it's a fake, this is supposed to be rounded.
5. On both a 57 and 58 unscrew the middle of the mic. There should be 2 wires one green one yellow. Sometimes counterfeit ones have the same colors sometimes not. If it's anything but yellow and green it be a fake YAR!
6. Look at the circuit board the wires are soldiered to. There should be a red control stamp, fakes usually omit this.
7. A fake mic will usually be lighter than a real one.
8. Authentic mics ship with accessories including a microphone clip, cloth cable tie, Shure sticker, carrying pouch, manual, and warranty card. Fake microphones tend not to include all of these accessories; most obviously missing is the warranty card and cable tie. Also, the bag will be of low quality -- sometimes with a "Made in China" sticker on the inside. Remember, Shure's microphones are made in Mexico, not in China. (Direct quote from Joe Shambro).
9. Lastly, trust your ears, the real deal sounds better. "
