How do I check the class of a micro sd card?
An SD Card class basically the minimum sequential write speed that the SD card supports. Testing the class of an SD is quite easy, just copy a 1 GB file to an empty card and measure the time it took for the copy. A class 6 SD card should support at minimum 6MB/s. 

Also check out the markings on the SD card, there should a logo like the following:



the number in the logo describes the speed class.