Where to go to see if my IP is blacklisted as a SPAM

MxToolbox has a web-based blacklist check tool that's a good starting point. The Domain Name System Blacklists (DNSBL) and Anti-Abuse Multi-RBL Check are other great free resources for this purpose.

DNSstuff also has a subscription based service called RBLalerts that will monitor 130+ blacklists for an IP address you specify and notify you if it has been listed or delisted. I haven't tried this service, but it might be worth a try if this issue is impacting your organization.

http://www.anti-abuse.org/multi-rbl-check/ is one

Although if an ISP is blocking you purely on RBL check then they need educating. You should never block purely on this as it can lead to a lot of false positives.

You should also check for yourself at rfc-ignorant.org. Being in their zones isn't being blacklisted, more like graylisted, but it doesn't help you any.