i'm not sure if there's a way to see if a site was once banned but if it is currently banned it should be removed from google's index and the pr bar should be grey or unranked
For checking whether a website is banned (assuming its not very new) use: site:domainname.com replacing the domain name with the website name.. If you see results then it has not been banned else it has been. If you are looking for google ban checker - www.seojunkie.com/2006/05/09/google-ban-checker/ has been around for some time.. You can utilize : www.whois.sc to see if it the domain was blacklisted. The method suggested by BrianR2 also works
Hey, thanks for all the suggestions and tips. I really found that my site was NOT banned after all !! just some indexing issue which has now been resolved. Thanks to everyone.