If not displayed press the "N" key to show the tool settings sidebar. Make sure you are in edit mode, and scroll down the sidebar until you find the "Mesh Display" tab. Click on it if the tab isn't already open. You should now see an area that says "Normals". Click on the faces option and you should now see lines coming out of your model in the direction of the face normals. By adjusting the "Size" slider you can change the length of the lines.

My Renderosity Store
Virtual Furnishing
My Portfolio