These are some basic tips to avoid buying the many, many fake Yonex rackets that are out there.
First, know the security features which Yonex incorporate into their rackets. Most rackets these days have a 2 sequence serial number. The second number gives the most information, as it gives the date when the racket was made. The first is the batch number. The numbers should be laser etched. If it's been printed on, then it's fake. If you're buying an expensive racket and want to make sure, you can check the serial number and batch number against the original Yonex series at badmintonforum.com, do a search under the Equipment Discussion section.
Muscle Power series or Armotech series will have features like the MP grommet strips (the black plastic strips along the bottom of the frame) and curved arches between the holes along the top of the frame, plus the CSC cap on top of the handle (an outwardly curving cone rather than the simple straight cone). Fakes tend not to have any of these. They just copy the general look and colours of the racket.
A genuine new racket will always have a tight plastic wrapping around the handle. They always have a bar code down one side. Beware of fakes that use just cling film, or loose plastic tubing.
If it's been factory strung, it will always have the Yonex YY log on the strings. Many of the top end rackets come unstrung from the factory to allow custom stringing so will just have a card insert. Beware of a racket with plain strings which is described as new.
Most fakes tend to be made for the Chinese market, so beware of any that have a CN country code. SP and TH are also pretty common for fakes. No-one seems to bother making UK fakes, so you should be okay with a UK serial number.
A more recent feature is the Yonex hologram sticker. These are little oval stickers, usually gold in colour, on the cone above the handle. I'm not an expert on these, but I do know that these are not added by Yonex themselves at the factory, but rather by the licensees (the retailers). I believe they were introduced by Sunrise Sports, one of the biggest Yonex retailers in Asia, as an additional security feature. Sadly, though, the counterfeiters have been pretty quick to copy these, so the presence of a hologram sticker is no longer a guarantee of authenticity. Also, the absence of one, especially on a European or North American coded racket, is no indication that it is fake.
Beware of auctions where the rackets are said to be based in Hong Kong or China, especially if they offer the racket at ridiculously cheap prices. These often have very large P&P costs, like a £100 racket will sell for £1, plus £29 postage. A fake won't be worth £5, let alone £30.
Lastly, some people may think that a fake racket is worth buying if you can get one for a fraction of the price of a genuine article, but it's a false economy. The fakes may look real, but they are usually of such poor quality as to be nearly useless. If you want a cheap racket, you are much better off buying a cheap racket from another brand that offers good quality rackets.
From my own experience, and those of other players far better than me, I would say the leading brands are Sotx, Winex and Gosen. Astec is also an excellent brand but very hard to find outside of Asia.
Hope this helps. Cheers, David