Just bought it on ebay from a chinese seller with high rating. The item description said it was GENIUNE. But I'm still skeptical. Even thought It works well on my pc the box looks suspicious on the backside. What should I look for to verify the authenticity of this controller?
Please take a look at these photos, and use them as an example if possible.
There are quite a few ways to check if you have a genuine Microsoft product.
The best way to tell is to check if it has a holographic Microsoft sticker on it. All genuine Microsoft products have this quality (watch this video for a demonstration of this)
In addition, you must remember that Microsoft has changed the design of their controllers over time, and therefore the one that you have may be different from the one your friend has. Wikipedia is helpful in identifying which controller you and your friend have.
The logo should look like this. The "Microsoft" logo on the top of the controller should have a split "o" and connected "ft"
Check the weight of the controller with another controller you are sure is real.