We use to use string transducers for that years
ago.. they were pretty cheap..we would zero them
at curb height with the driver... they would measure
either + or - from the zero so if it went into compression
it went - or + on extension... when I says cheap they
were a little over $100 back then... if its only going
int compression thats easy to check..a zip tie around
the shaft.... I dont know what they use now days on the
shock travel sensors... a camera under there can tell
you a lot also