[Justin Stiltner]

with the sensors you have I think the most obvious one is:

You know the potentiometer value for a gripper that is closed with nothing in it(object of zero width) and you know fully open. If you command your claw to close, it does so, and the steady state value achieved is not that of a zero width object. Then you know you must have grabbed something, and your claw is no longer moving(steady state) so you change from your closed loop position controller to just applying some small amount of motor pwm signal that is enough to hold the object.

This works in this case because the motor is stalled and the pwm in effect controls the voltage, which will be proportional to the current which produces the torque in the motor.

alternatively you could have the set point for the claw position controller be set just a bit more closed than the steady state value, assuming you are only using a proportional controller this will have the same effect as setting a small pwm value as previously stated.