Mellotronists

OK - before I replace these worn out tapes, how do I know if I did a 
thorough job demagnetizing the tape heads?  I know there's 
magnetometers (gaussmeters) out there, but in fact, has anyone 
measured the magnetic field strength of a tape head when it's 
magnetized and/or after being demagged?

(Just being paranoid - don't want to wipe the high end out testing my 
work).