In WindO$e 7 try to copy any 200MB file onto your microSD card, and expand that copying files window to show the speed of data copying, if it is around 3 --> 4 MB/s then it is a class 4 card.

I bought an unbranded 2GB microSD card with no labeling on it with a very very very cheap price, and it turned out to be a class 6 card, not bad.