Take a wheel off your car. Lay it on its sidewall so that the inside of the wheel (the side that faces the hub and brakes) faces up. Lay a straight-edge or something across the rim (not the tire). Measure up from the wheel's hub-mating surface to the straight-edge you laid across the rim. Subtract 1/2 the wheel's width from this measurement to get the wheel's offset.
A wheel with 0 (zero) offset has the hub mating surface exactly in the center of the wheel's width.
A wheel with a positive offset has the hub mating surface "offset" to the outside of the wheel (bringing the tire in
A wheel with a negative offset has the hub mating surface "offset" to the inside of the wheel (taking the tire away from the car and closer to the fender).