Detecting the Underage use of Magic

[jimlaming]

No: HPFGUIDX 97144

I have been wondering about the MoM ability to monitor the underage 
use of magic.  There is a great thread at #94458, which discusses 
mostly the age aspect of the issue.

What I want to know is: How do they know what magic is being done and 
by whom?  Is there any logic or intuition or analysis that is applied 
to the detection and the resulting violation?

There was a reference to tracking wands in that thread, but the 
unintentional, emotional magic that resulted in Aunt Marge inflating 
was wandless.  When Fudge catches up with Harry at the Leaky 
Cauldron, Fudge knows all about the act of magic and he excuses 
Harry.  Why did the MOM, and thus Fudge, know the circumstances of 
this magic but not know the circumstances of Dobby's involvement in 
the hovering charm a year earlier?  

When the Dementors event happens, the MOM is all over Harry.  Now I 
know Fudges' opinion of Harry has changed dramatically, however does 
anyone at the MOM know the circumstances of the Dementors event 
(besides Toadface)?  Is there a record made of the underage use of 
magic detection process at the MOM?  Could we go back in the logs and 
find it?  Could Toadface alter the record?

Jim, who loves asking questions, even stupid ones