Q. How do I find out which corporations have been delisted?

Go to Bloomberg (available in the Humanities/McLennan Library, 3rd floor (Room M3-53B)). 

Type: CACT and hit GO
Click on Action Types-->Corporate events
Select [Delisting]
You can change the years in the amber fields.

In addition, for Canadian companies:

See the monthly publication Toronto Stock Exchange Review. Look for Deletions in the Changes in Stock Listing