Simply measure from the top to the bottom of the carriage bars (plates). 

Typically, they break down as:

Class 1 = 0-1,999 lb capacity truck

Class 2 = 2,000 - 5,500 lb

Class 3 = 5,501 - 10,000 lb

Class 4 = 10,001 - 17,500 lb

Class 5 = 17,501 - 24,000 lb

However, measure before ordering since these are only guidelines. There are various reasons manufacturers may use a larger carriage, particularly on trucks that are close to the larger capacity. Measure once and only have to order once.

For additional carriage dimensions, check out ITSDF (Industrial Truck Specification Development Foundation).