Calculate the time of your local noon for your location
http://en.wikipedia.org/wiki/Noon
http://education.illinois.edu/noon-p.../noontime.html
Then, use a piece of string, with something heavy dangling on the on the end.. Place it (at noon) such that you can clearly see the string shadow over the place where your slab will be (or already is).. mark that direction/line clearly.. and that's about it.