Why does it matter?
Genuine Mugen covers are flame retardant (aka "fire proof"). This is a safety feature that the fakes do not have.
How do I spot fakes?
1. Photo shows no packaging, but seller says they are new. And seller also offers unpackaged, brand new reservoir covers from other manufacturers.
2. Picture shows packaging, but item is being sold by a store that is not known to sell other genuine Mugen items. Fakes can have basic packaging, but can be missing the rubber retaining ring or the printed instructions on the back of the card.
3. Price seems too low to be true.
4. Mugen part number is missing from the package.
5. Seller has many reservoir covers for sale, but this is one of the only Mugen products they sell. Beware of sellers that only sell some combination of reservoir covers, pedals, oil caps, shift knobs and radiator caps -- the most commonly produced fakes.
How do I buy genuine?
Obviously we recommend that you to buy from King Motorsports, where you can rest assured that your Mugen products are genuine and made with the correct materials and quality control that Mugen intended. If you are buying an item second hard, follow the same caution you would with any other purchase. Ask to see photos of packaging, if they have it. Ask if they have an original invoice -- many private sellers of second-hand items within the USA often have boxes (applies to larger parts boxed up in cardboard with original Mugen stickers) and invoices that are printed with KING MOTORSPORTS on them.
Pics of genuine Mugen reservoir covers courtesy of H-T member EFcivicJDM: