Somewhere on the body of the coupling (not the coupling head), there should be an ID plate, some examples of the ID plate location are shown below. This plate should contain the manufacturers name, a type number and weight of the coupling. This is often most of the information that is needed to figure out what parts are fitted.
These are just guidelines; over the years different numbers have been used.
If you still need help, please don't hesitate to email us with as much of the following information you can: Coupling make, coupling model, coupling weight and parts needed, photos of the coupling can be every useful as well.