> how can I determine if my rotor has lost some of its magnetism ?
Test if I can dangle a huge/medium or small spanner (s) from it ?


It's the other way around, normally.

If the rotor dangles from the spanner without any hint of dropping off under its own weight (test all magnets) then it's still usable but may well have lost at least some of its original magnetism but difficult to tell without comparing it to a new one.