Use a caliper (a thickness measuring instrument - not the brake assembly that straddles the disc ) to measure its thickness. The minimum thickness is stamped on the rotor. Note that you should factor in the rotor wear with your new set of pads. I've found that the fronts won't safely go past 2 sets of pads, but the rears will be fine with 2 sets of pads. In other words, replace the front rotors with every pad changes and replace the rear rotor every other pad changes. That's my recommendation.

BTW...my observations are with regards to OEM pads and rotors. Other brands or types may varies.