I am pleased that you managed to rescue the situation, that is one mistake you will not make again.

One of those things that comes with experience. I have several easy methods to make sure I don't do this but I deal with magnets daily for other reasons. I find the simplest method apart from the fluxmeters and search coils is to hold a decent sized neo magnet in the fist and pass it over the mounted magnets to see that it alternately attracts and repels. As long as you have the thickness of your fingers between the test magnet and the rotor you are checking it is perfectly safe, but even so I suspect someone not understanding exactly what to do could screw it up and hurt themselves so use your own discretion.