usually wheels have them stamped on the inside of the wheels. you can also check the distance on the inside of the wheel from the mounting surface to the outter edge of the wheel.
Here http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101 That explains it decent. Most rims are a positive offset, which means the centerline of the rim is pulled to the car more. As offset goes down in size the centerline of the rim gets moved out away from the car. On our trucks a a 50mm offset leaves a space between the tire edge and the fender lip. When you bring it down to say a 30mm the gap between the tire edge and fender is much smaller.
