i think the quickest way to find out is to type out the URL of the site into the search engine...if nothing comes up it's probably de-indexed... Another way to find out is if you have the google toolbar, if it shows all gray for a particular website I think that means it is not indexed and possibly banned...
www.iwebtool.com << there is a tool which tells you whether or not the site is banned. edit: exact url :http://www.iwebtool.com/google_banned
Back links drop drastic ..and you can try site:yourdomain.com .. but it may still find a few straglers. First...Your index and main pages will drop and then backlinks and then eventualy not a dam visitor or link to find you except for obsucre stuff that floated to the top. I had a health site with to many links (unrelated) so they droped it like a ton of bricks with 10,000 backlinks. took about 3 months to get ride of it form the search. But they did.. lol Hope this helps.
Definately the easiest way is just going to google and search your full url. Example: search www.yourdomain.com If your site doesnt' come up, that means that the site has been dropped from their index.