this is where we say to post a link to what you have.
Based on your post, I would assume that any device you have is a single channel and fed with mains voltage.
This is not a good candidate to tap into, especially since you said that warranty might be involved.
but, using some dedution, when powered, the devie would drive to an end (or not) and while powered, the signal would be present. while present, you can detect it. simple to do.
here is a deduction. once an end-switch has been reached, power is lost and the device REMAINS in place until power is applied to drive it the other way. this is an assumption since we do not know anything about the thing.
based on the above, you have to run wires to the switch, need to make a circuit to work from mains power, and probably use a optoisolator to protect your Arduino.
since you have to run wires then post #6 becomes the safest and least intensive interface. a glue-on sensor would not interfere with the device and should have no impact on warranty.
another option might be an optical sensor. so that at full extension or full retraction, a painted target would be seen and the sensor would be separate from your device. I like jremmington's solution better as over time, dirt could effect the operation of a visual sensor.