As I recall, the N3D stylus tracked at about 6 grams, so using the finger test for compliance, you won't feel as much movement as with modern styli, but there should be some. If you feel none at all, then yes, the stylus is shot.
The N21D stylus was a lower-VTF replacement. There was an N7D as well. I don't know of a source of original Shure styli (possibly Garage-A-Records), but for 1960-level tech, I really think aftermarket replacement styli will do the job well enough.
I was hoping JICO made an SAS stylus for this cartridge, but darn, no luck.
A lot of people really like the jukey sound of these old monsters. A fan of this cartridge may very well happen along and fill you in on the optimum care and feeding of the M3D. Meanwhile, I'll continue to try to find a catalog entry for this Radio Shack clone.