There are a range of ways this has been done. From the simple to fairly complex.
For an example of a simple one have a look at this thread,
http://www.cnczone.com/forums/showthread.php?t=36099 (Another Aussie Auto Tool Zero Setter)
And here is a demo of this and a couple of other ideas
I use this on a router, engraver and mill.
"http://www.youtube.com/watch?v=Oc-9gTFj-y4"]YouTube - Laser Zero finder