A magnet supplier once gave me what looked like a credit card. Instead, it is a laminated transparent sleeve containing iron particles so fine they aren't readily descernable. If you hold this card near a magnetic field, it displays the flux. I have found it invaluable in mounting magnets in outrunners. I'm not sure where these are available if they still are, but it would be interesting to hold it over your magnets just to see what the field looks like. It can spot a cracked magnet in an instant.

The hall sensor paper is well done, I'll keep it as a reference.