There isn't any good way to measure magnetic field strength for the layman.
Actually, we do have access now to very reasonably priced Gauss meters like this one from Hojila.
https://www.ebay.com/itm/394645582845 I don't see a minimum frequency spec. so I assume this gadget can measure static magnetic fields. EMF meters can quantify magnetic fields, but generally have a frequency range that they are calibrated for, and don't give useful readings in a static field.
Or, one could cobble up something that can quantify a magnetic field by rigging up an auxiliary circuit for their multimeter, based on commonly available packaged Hall Effect sensors, like is described in this science experiment.
https://www.sciencebuddies.org/science-fair-projects/project-ideas/Elec_p030/electricity-electronics/measure-magnetic-fields#:~:text=Once%20you%20have%20assembled%20your,the%20front%20of%20the%20sensor).
And, any DC clamp ammeter, like the UNI-T UT210D, that I've been recommending for general purpose moto diagnostic work, has a built in Hall Effect sensor in the jaws that may, or may not give useful results when exposed to a magnetic field in a systematic way.
One could rig up something even simpler, like a pull force test on a piece of iron. But it's not THAT simple, because the pull strength varies non-linearly with distance, and the deflection during the test needs to be practically nil. ( This suggests another, remote possibility, that the rotor/stator fit is incorrect, and there's too much gap between stator and rotor. I have no idea if different diameters exist, though)
The rub of testing a questionable rotor is the calibration. If rotor specifications EVER included the expected intensity of magnetism they were supposed to produce, it would have been over 100 years ago, at the dawn of automotive electrical systems when they had just a couple of poles. We don't know what the field strength of a good RE UCE rotor is supposed to be.
The first step I'd have to take in measuring the magnetic field of a questionable rotor would be to acquire a presumably good rotor. Because I'm methodical, the next step would be to mount it on my engine, and verify it produced the specified output in service. Only then would any magnetic field readings taken from it have much usefulness, or be worth taking. Obviously, most people would stop at the point they had their new rotor mounted and pumping out electrons, and save themselves the cost of the gaussmeter, and the time spent taking readings and making sense of the results.
Such experiments only make sense if I expect to encounter multiple dubious rotors in the future, or am part of a larger community of geeky science /engineering nerds, who are acutely interested in the results. I don't think either of these is true.
If I'm wrong, and people are acutely interested, here's a range of magnetic field strengths one might have available in their home shop.
0.5 Gauss – Earth’s magnetic field at its surface
100 Gauss – A typical refrigerator magnet
1,100 Gauss – Magnetic rubber grade Y
3,700 Gauss – Ceramic magnets
11,000 Gauss – Samarium cobalt grade 2:17 magnet
12,500 Gauss – Alnico grade 5 magnet
13,000 Gauss – Neodymium grade N42 magnet
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