I have now witnessed three different types of Sennheiser E845 fakes (two of which I have bought on EBay). Here are some tips to avoiding them that have not been previously discussed on other posts.
Weight: An 845 should weigh 330 grams which is over 11 ounces. Many of the fakes weigh around 8 ounces.
Impedance: 845's are 350 ohm mics. Most fakes are 600 ohms.
O ring: A Sennheiser has a rubber O ring on the body at the base of the windscreen. Most fakes do not. Unscrew the windscreen and look at the point where the windscreen contacts the body at the base.
Balanced/Unbalanced: Some of the real crappy fakes are not even differential (balanced outputs). If you have a volt/ohm meter check continuity between the pins. There should never be two pins shorted together. Some fakes are cheap unbalanced outputs (but some are not!). The audio quality on the unbalanced version will sound quieter and more midrange heavy compared to an original.
Pickup pattern: Some of the fakes actually say "Unidirectional" on them if you unscrew the windscreen. This is a dead give away that is is fake. An original is Super Cardiod.
Switches (845S only): Some of the switches have "fake" molded in set screws.. Again some of the better fakes actually have set screws so buyer beware. Also, the original switches are high quality and should not feel "mushy" or "drag" when slid.
If you buy a fake, Sennheiser has a page on their website to report fake microphones. If you pay via PayPal online they have a claim process to get your money back. At this point I am coming to the conclusion that there are as many fakes as real ones out there on Craigslist and EBay. Sennheiser suggests checking their posted authorized dealer list before purchasing.