If you look on the SD card itself, it should have a number in a circle, that is the class of the card. Class 2 (for example) means it can sustain a 2 megs per second write speed. Read speed is almost always faster. There is also response time which is preferable to be under 1 millisecond.
If you run a speed test, the upload speed determines the class. Be warned though, many card readers limit your speed and a speed test may not always be accurate because of that.