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Offset is normally stamped or engraved into the wheel and is measured in millimeters of ET: ET is the short form of the German word Einpresstiefe which translates as insertion depth.
Offset is the distance between the hub mounting face at the back of the wheel and the wheel's centerline.
Negative Offset wheels have their mounting face toward the rear of the wheel - powerful rear-wheel drive cars often have wheels with negative offset.
Zero Offset wheels have their mounting face even with the centerline of the wheel and are by definition ET0.
Positive Offset wheels have their mounting face toward the front face of the wheel. Front wheel drive vehicles usually have positive ET wheels. Eighties & Nineties Volkswagen wheels are generally ET38.