How can you tell??
Early versions of the OM40 had the Van Den Hull stylus, later ones had the Fritz Geiger, they are both excellent (and the VdH is made by FG for VdH), but it is one way of identifying.
Early OM30's were also available in an OM30H (high compliance) version, I presume the stylus would have been the Stylus30H...
This is the sort of thing you have to get lucky to find (and have to be looking for it!)