Here are some basic indications shared by websites that have been blacklisted:
Your Site is diverting to a porn site, Japanese site, fake merchandise site, credit site, or some other variety.
Your Site shows a ‘This site has been suspended’ message. Likely your host blocked your site so check your emails (and spam folder) to see if they mailed you about your site being contaminated.
You see pharmaceutical references on your site.
You see new pages on your site. Keep a eye open for pages that look like login pages for the major banks (i.e., Bank of Toronto, Pursue, Wells Fargo, and so forth.)
Customers are grumbling that desktop antivirus projects are stopping them from accessing your site (i.e, McAfee, AVG, Microsoft, and so on.)
You see owners in your Google Webmaster Tool that you didn’t make or approve.
You see new admin users created that you don’t in your administration dashboard