The simplest way to do this is to follow the steps below;
Step 1 - Let your beard grow for a couple of days.
Step 2 - Print off the above photo.
Step 3 - Run your fingers over your stubble and take note on which direction you feel the most resistance. Lets say you feel resistance when you run your fingers upwards, you then know your stubble grows downwards.
You can also use the interactive face map here: http://www.pbjsite.com/shave/whiskarmap/
Don't be surprised if your stubble grows in various directions or changes direction, this is very common. Always remember that we're all different. If you are like me and you cant shave against the grain then face mapping is essential and you should take the time to get to know your face.