How many centimeters of mouse movement your full 360 turn takes - the truest measure of sensitivity. Free.
The distance in centimeters you move your mouse to spin a full 360 degrees in-game. It is DPI- and game-independent, so it is the best way to keep the same feel across games.
Most FPS pros sit between 25 and 45 cm/360. Lower is faster, higher is more precise.