I use a standard edge finder... Like this:

This one being a starrett.
Snap, except mine came from RDG Tools (IIRC) and didn't cost as much as a Starrett.
They are available in imperial or metric sizes, I bought imperial. Since I have a DRO, I usually edge-find the work in imperial, then reset to 0,0 & switch to metric if required. If you work is mainly in metric AND you have a DRO, then you'll be easier doing it the other way around. They really are easy to use, and surprisingly accurate, so long as you're patient.