Thanks to those who responded to me off list: davidhouck, aadrian and Mica. Bless you all.
I tried each of your various suggestions:
a) use iron filings and a piece of paper to try to see the magnetic lines of flux, see if there is a difference [had no filings, used steel BBs instead; no luck]
b) see if there is a difference in output from the MXYs [Yes! O, yes!] and between the FatBoys and the Series pickups [Sorry, none in volume and very little in sound].
c) see which PU produces more noise, especially if a cell phone is dialed nearby. [Hmmm... they all did, no discernible difference there, but perhaps my amp wasn't hi-fi enough to reproduce it. Wow! These PUS are all REALLY sensitive!!!].
Finally, I got to wondering if there was a difference in weight, as the Series PUs are single coil and the FatBoys are dual coil. Turns out I was correct. The FatBoys are just a bit heavier [Sorry, I can't give you exact figures. I used a balance scale with one pickup on each side. Nevertheless, there was a clear difference. YES!].
Anyhow, thanks for all your input and, for those who have been wondering, that's how to tell the difference between FatBoys and Series PUs (assuming you have one of each to check].
The best method, of course, is not to mix them up in the first place.
This thread probably ought to go in another forum for future reference, but I don't know how to do it. Perhaps one of the moderators could help out here?
Thanks again, everyone! Case closed.
Dr. J