Current is proportional to torque, so you should be able to measure the current and calculate torque. You can estimate the maximum torque that each motor should provide, and use software to limit the output based on that.
From my basic knowledge and a
brief reading of the
Wikipedia article on ABS, the basic idea of ABS is to measure the speed of each wheel, calculate the overall speed of the car. If one if the wheels is significantly slower, it is about to lock up. Conversely, if one if the wheels is going faster, it has broken static friction and is spinning. In the case of a tank-steer robot where each side of the drivetrain is connected, (rather than using a differential and having independent speeds for each wheel), you would probably need to use other sensors to measure the robot's actual speed.
I'm sure there's a lot more going on in modern ABS and traction control systems, but several teams implemented such systems on their robots in 2009 with demonstrable success. Search around and you should find some discussion and video.
All that said, in the context of FRC, having your wheels slip is not necessarily a bad thing. Given the choice between having your wheels slip and tripping a breaker, you definitely want the wheels to slip.