Question about exception specification

[Graham St Jack]

I read WalterAndrei.pdf recently, and liked most of what was in there.

However, there was the line: "Exception specifications: bad idea" on a 
slide that discussed a possible nothrow keyword.

Can anyone explain why they are a bad idea? And if they are in fact a 
bad idea, how do I find out what exceptions can be thrown by a function? 
Eyeballing heaps of code doesn't count as a way!