Few ways, the method I use and the one that will give you the best tools to optimize your gear and to get others to help you optimize you gear is sign up and put you, build, gear and datacrons on Ask Mr. Robot.
Bremiaha
Mar'ina
Another way to do it is just look at you gear and add up the numbers. Old fashion, but still works.
In game way is go to the Character Sheet hover mouse over Critical Mutiplier.