Do you have a freewheel that you know to be English thread? If so, you can thread the non-drive side cup into it to check the thread pitch.

From Sheldon:

"A bottom-bracket cup can serve as a thread gauge for a freewheel: Dimensions are usually marked on cups. Hold the threads of the bottom-bracket cup against those of the hub, and look in between, against the light. If the threads engage tightly all the way across, the thread pitch is the same. If they rock across each other, it is different. You may check the thread pitch of a freewheel by threading an ISO left bottom-bracket cup into it (not a right cup, which is left-threaded). The cup will go in easily if the thread pitch is the same -- but do have a freewheel extractor tool handy so you can unscrew the cup."