If your belt is a uniform color then you can use contrasting paint to mark a line and sense is with a photoresistor. An example is a white line painted on a black belt. There are tons of examples online for line-following applications. The circuit uses a photoresistor or phototransistor in a voltage divider. To make it even easier you can get a pre-packaged unit like this:
http://www.acroname.com/robotics/par...E-LINE-IR.html