How would you know whether a site is banner by Google? Does it just disappear from the search results? Or do you get a notification?
Search for yourdomain.com and see if your site shows up. If it doesn't then you've been deindexed. If it shows up but your rankings have taken a huge dive then you probably just have a penalty. Do some research on the -30 and -950 penalties to see if you have a penalty.