I decided to add to the other guide somebody else already wrote here because there are some things that are either missing or the fake headsets got more sophisticated and they are harder to spot.

I bought a fake headset at ebay (for $50, damn!!@@!) However I didn't know it's fake till I came across an article about how many fake goods are sold at ebay. Astounding! And Ebay is hardly doing anything. (Perhaps even not in positition to do anything).

My first doubt came when I realized the headset had some small issues and I was quite sure Motorola wouldnt just leave these things on a headset that costs over $130 in BestBuy.

Anyway, I decided to purchase another S9 from Best Buy. Came home and shock. Here is what I found:

FAKE:

- Motorola logo surface is just above the plastic frame (doesn't sit flat inside). The logo has clearly areas of poor manufacturing, the circles forming the logo are not clean and the logo is not clean cut/molded either.
- the battery compartment has a strange irregular finish. Not matt, not glossy. Just like a cheap plastic.
- batter compartment "red rubber". You can feel this difference. The fake headset feels like a silicon keyboard cover or rubber on your shoe and easily collects dust from your fingers.
- charginig jack - you can see the mini USB jack is straight, not in like with the arch of the headset's battery compartment. This is because the jack is mounted on one circuit board that is sitting flat to the battery compartment.
- the power on/off button is elevated, looks ugly, obviously covering a cheap switch underneath. This switch produces a subtle click.
- the glossy black plastic rings connecting "arms" and battery compartment are irregular shape, not clean "cut", finish is not glossy as it would be typical for this kind of material and design)
- the "arms" - here you can see the sharp edges of the molded parts.
- buttons + - > < have obviously mechanical switches underneath (which definitively would fail soon). You have to hard press to do anything. You can hear a click (+ tangible feedback) which shouldn't be there
- indicator LED light - only 2 colors - blue and red, no green ligt. The plastic LED is not sitting properly. Looks like a not fitting part.
- rubber marking "arms" - the rummer has sharp edges (like unfinished)
- Headphones buds - when looking from the top, you can see the mesh. Mesh on fake headset is approx same glossy as real headset, but it is uneaven. It shows some irregularities.
- When you remove the in-ear rubber rings the rings sitting next to the arms of the headset are rubber and are easily coming off. They are not attached.
- In ear rubber rings are of lesser quality.
- Music quality - as the other guy already said - night and day - but you won't realize unless you really are an audiophil or you try both fake and real and compare them.

REAL:
- Motorola logo nice sitting flat, good quality.
- battery compartment regular finish, more matt than glossy.
- rubber part of the battery compartment is smooth (like a good quality condo$, if you know what I am talking about)
- jack is in line with the arch
- power on off button is flat and you can hear audible click when pressing it.
- plastic rings are glossy
- arms are very clean cut
- buttons are touch bottons, so there is no click
- LED has 3 colors, red, blue (which is brighter than in fake) and green (normal light)
- rubber on "arms" is round, pleasantly cut
- headphone mesh is regular, you can see the quality difference.
- the rinigs next to arms are firm, not removable.
- the in-ear canal rinigs are of a better quality, the small ones are firmer than fake one, the biggest is softer than fake one.

The majority of S9 headset sold from Asia are FAKES! I did a bit of research and I was almost sure my headset is real. Despite of proper due dilligence I was victim of a fake one.
In overall - if I can give you an advice - DO NOT PURCHASE Motorola S9 headset from Ebay unless you can pick up and see the headset in person. It is not worth it. Take care.