How to identify real apple 20W (3 pin UK) power adapter?

Hi,

I have seen several 20W power adapters in the online Markets. I wanted to order a 20W power adapter through an online shopping mall. How to identify real 3 pin UK power adapters by looking the box and the model number as the only details I can get from the seller are pictures of box and model number? Any other way to identify with out opening the box? If the seller give apple care warranty, will it be a real product?