There is only one real way, unless you bought it new, when it will say the guage on the box. With second hand models, if you measure the distance betweren the weels and divide it by the actual size,...
The name comes from the German groß (meaning "big"). Traditionally, G scale is the use of 45 mm (1.772 in) gauge track, as used for standard gauge (Gauge 1) models, for modelling 1,000 mm (3 ft 3...