Any product bought from any website should really be tested unless you are 110% sure that the product is legitimate. You should firstly do as follows...
1. Download a software package for free called H2testw v1.4 (Google it, it will come up)
2. Install the software and plug the flash memory card in your computer and run a full test.
3. Once the test is complete it will tell you if the card is real or corrupted(Fake)
4. The mb/sec will be there too, which will indicate what class it is i.e. A Class 2 writes at 2mb/sec, class 4 is 4mb/sec, class 6 is 6mb/sec and class 10 is 10mb/sec+ (In approximately 1% of cases, some laptop/desktops running this test at about a 50% lower write speed, i.e. a card which runs normally at 15mb/sec the software says around 6mb/sec, you should always test on another computer to confirm.)
5. See below a fake card, the software states that the card has 1.9gb of data and 29.3gb was data lost. This is 100% fake, and you should report this to the advertised manufacturer via email and eBay or Amazon if bought it from there site.
7. Remember even if the results are real like the below, and you are unsure about the authenticity you should always email the manufacturer, and they will confirm this.
9. Remember that it is the writing speed which determines the class of the card, not the reading speed. I have seen a seller advertising Samsung micro SD cards as a class 10 with the H2testw software screen shot with a write speed of 6mb/sec. These are fake/counterfeit, and Samsung would not develop a card of this standard. One of the best one I have seen to date was a seller advertising a SanDisk 32 GB Micro SD Class 10 cards, however SanDisk do not even manufacture a standard black Micro SD card class 10. The seller of these cards was an eBay Top Rated seller..... Basically be careful what you buy, do your research first.
I hope this information helps.