It depends on the brand, but sometimes they have a number on the face contained within a circle or partial circle. Sometimes they have a name like Ultra or Ultra II, which identifies the speed. Here is a link to show how to tell the class, but not all cards are marked:

http://www.sdcard.org/developers/tech/speed_class/

I think the speed per class is as follows:

Class 2: 2 MB/s
Class 4: 4 MB/s
Class 6: 6 MB/s
Class 10: 10 MB/s