Every once in a while, maybe every few days, I pull up my bookmark for the SBL Latest Additions and Removals over on spamhaus.org. It's usually pretty interesting to watch. It's basically the only public announcement that a new sender, ISP, ESP or ESP's client has gotten blacklisted. If you keep a close enough eye on it, after a while you'll start to get a feel for which ESPs and ISPs have the most ongoing issues or perhaps the most permission-challenged clients.
If you're really fancy, you'll figure out that you can bookmark links to various domain name groupings of SBL listings, allowing you to check for listings relating to the network of a particular ISP or ESP. Here's an example.
I'm not the only person who keeps an eye on these, am I?