Can I measure if current is on or off in cable with 24v DC (in a cheap way)
from what I've read of these they get current that makes them hot then their inside swells up and pushes on the pin, and also switches them to a lower power mode where they use less current.

it looks they are normally powered from 24V AC

for determining their operation you've got two options, either you fit a split core current transformer around only one of the conductors in the cable,(brown or blue doesn't matter which)  or you connect a voltage sensor (eg: optocupler arranged for 24V AC operation) across to the same terminals that the brown and blue wires go to.