Surest way to tell is to remove one of the injectors. If it has one discharge hole, it's a 30 lb. injector.
If it has two discharge holes, it's a 36 lb. injector.
If it has three discharge holes, it's a 42 lb. injector.
This applies to the "TOMCO" branded disc injector with the green stripe at the neck of the inlet portion on the top.