Here are some tips to prevent you buying a fake Shure SM57.
Firstly, the mic should weigh 284g. Fake will weigh approximately 240g.
Secondly, if you have a multi-meter check the resistance between pins 2 and 3. It should be no more than 400 ohms. Fakes will be approximately 553 ohms.
If you are in any doubt, you can contact
Technical Support Manager
Shure Distribution UK
167-171 Willoughby Lane
London N17 0SB
Tel: 020 8808 2222