While typing speed is not the most important transcription skill, a few companies do require minimum typing speeds. Additionally, the faster you can type, the more money you can usually make, although money made with any given transcription job is greatly affected by audio quality, the type of material being transcribed and your familiarity with that material (and how much research is necessary if you are not familiar with the material).
These are some free typing test and/or practice websites.*
http://www.brainbench.com (has many tests that are not free and some that are free, including a typing test)