Jeff has an excellent answer here with the reference to mouse-sensitivity.com, but let me explain the method behind that site so you have the alternative of doing it the old fashioned way.
Get a ruler or tape measure and put it in front of your mouse, horizontally across your mouse pad. Open a game that you already has your sensitivity set to what you feel is comfortable, and what you want to duplicate in other games. Open that game up and spawn into any map where you won't be harassed for a few seconds.
Make sure mouse acceleration is turned off. If you're using mouse acceleration, just don't. It's killing your ability to aim. If you're using a Mac, which has acceleration locked to on, then I weep for you. You'll need to find a utility to turn it off.
Now, face a distinctive point in the map; something easy to recognize and get a fine aim on. Texture details that form right angles are good for this.
Put your mouse on the left side of your mouse pad with the left side of your mouse lined up with the zero on the ruler.
Turn to the right until you've spun 360-degrees and are pointing precisely at the spot you were before the turn. Note on the length on the ruler.
Repeat this process in a second game, adjusting your sensitivity until you match the same distance as in the prior game.
Average competitive 360-turn distances are between 15-50cm, with 15cm being considered high sensitivity and 50 (yes, that's half a meter!) as very low. I tend to run in the 20cm range myself.