a comparator circuit is what you need. It has 2 input terminals, one (-) goes to the voltage your detecting via a potential divider,
the other (+) goes to a reference voltage (can be produced by a zener, or a more accurate voltage reference).
The potential divider you want to be adjustable, its purpose is to divide the input voltage down to close to the reference voltage
on the other terminal.
You can switch (+) and (-) terminals around depending on what output behavior you desire - whether it goes high or low when the voltage
threshold is crossed.
you can do this comparator with an general opamp, eg LM358,LM741 etc or a comparator device LM339,LM301 etc
google these, find heaps of results.
oh, one more thing, I have done exactly this sort of thing in the past, do not underestimate temperature variations in the
voltage reference effecting the threshold you set in a automobile application.
If using a zener reference, use 5.1V as that has the minimum temperature coefficient, or really recommend a reference like LM335.