Mugroop's Syndrome is one way to test the level of background magic in an area. Cerebral manifestation gives a rather large clue to there being a high level of magic. During the events of Interesting Times, when the Faculty of Unseen University approaches Fourecks where the background magical radiation is over a million thaums, the Bursar develops Planets. They are insubstantial and go through him and through each other, but they are nevertheless there.
Old 'Dicky' Bird was a Wizard who was better than a canary down a mineshaft. He was very susceptible to high magical fields. He'd doze off, and when he woke there'd be... we never find out exactly what due to the bickering that ensued, but from the context it was probably avian.
For more random wizardly goings-on, see here.