how do you know if you've been deleted?

you can go to the profile page of that person. if you see send a message as a box below the avatar, it means you are still in the user's list. if add as friend is written, then you are not his friend.

I guess you would know if you tried to instant message them. It may not go through if you are not on their friends list anymore. That is one good way to check it. Have you tried to do that yet? I had one friend who harrassed me and I cancelled their friend request afterwards but they still contacted me though a PM, so I am not sure.

well if you don't get their notifications anymore, that is one way, or you can go to their home page and if you do not see your name appear as their friends anymore, you have been deleted by them.

I reckon the simplest way is to not notice their name on your friend's discussion lists anymore - though you know they have been active with starting discussions - you can also check their friends list to see if your name is included - or your own friend list to see if theirs is still in it.