try to google 'related:your-domain.com' to find related sites (from Google's point of view) and http://www.domaintools.com/reverse-ip/ to find out what sites are hosted on your IP
When you see a forum full of "buy viagra" spam - that is a bad neighborhood. When you see a directory full of "buy cheap rolex watches now" links - that is a bad neighborhood. Sites that say - watch stolen copyright protected videos now - that is a really bad neighborhood. A bad neighborhood is anything full of spam or promotes copyright infringement. This includes blogs, forums, directories,,,, anything anything where the admin does not keep the spammers out.
There is a BIG difference between sites that are for advertising and sites that promote spammage. Lets take sites that sell prescription medicines - By selling a prescription medicine to someone that does not have a doctors prescription - they are breaking the law. That means that all of those "buy viagra" sites are spammers. Take sites that promote copyright infringement like videos and music - they are breaking the law. Then there are the scammers - the people that sell you something and then never deliver the products. There is a big difference between a site like Amazon where you can buy a DVD movie and they ship it to you. And a site where you order a DVD and never get it. Scammers and spammers are bad neighborhoods. Any site that has a lot of links going to spammers and scammers are bad neighborhoods.
Bad neighborhoods are sites with excessive links and have worms or viruses (scripts) in them. There are also sites that are classified as bad neighborhoods because of their IPs.