Also, if one of your assassins has been ordered to assassinate but there was no assassination attempt, you know that there aren't any commanders left.
So, if you have two assassins assassinating in a single province, and one of them assassinates a commander and the other doesn't, you know for a fact that all commanders are dead.
You should notice though that assassinating commanders only works well against indies, because a normal player (or AI) can use his PD to make your assassinations useless.
Wow, so many S's in a single post :O
And, a funny quote related to assassinations:
http://www.bash.org/?73185