You can try looking up your ip address to see if its blacklisted http://www.mxtoolbox.com/blacklists.aspx
You can try your IP http://www.whatismyip.com/, and also look in the headers of an email that you sent to see other IPs that your email is using.
Some of the blacklists web pages will give you information about why you are on the list and also give you an option to request to be removed.