I have an external limit switch, that when pressed, will pass 12V. When not pressed, it passes 0V.
Not good enough!
From your further description, the limit switch actually disconnects
the motor when pressed, so you need to explain why it "passes" 12 V when pressed. So you actually need to show us (as described in the instructions
for posting images) a complete circuit diagram
of how the motor and switch are wired.
My last attempt was to connect the 12V lines (Pos. & Neg. ) from the switch to a ACS758 board and the corresponding vcc, grn, output to Arduino. I got nothing of value on the Arduino analog pin... and the motor attached to the limit switch continued to run when the limit switch was pressed.... when prior, it stopped running as it was supposed too.
The ACS758 is a current sensor; connecting it in parallel across a switch will effectively short circuit the switch preventing it from turning off. You could
use it in series
to determine whether the motor was running or not, but that would still not tell you whether it had stopped due to the limit switch or any other control. Unless you provide a complete circuit diagram
of how the motor and switch are wired, we cannot advise on how to correctly sense the specific
operation of the limit switch.
A more detailed description of the purpose of your complete system would also assist. This begins to sound like an XY problem