I can't agree more with Gary, we probably need some more information to know what kind of black hat you're possibily dealing with. One of the first things I tend to look at trying to find out if the site is using some ways of black hat tactics are:
- Backlink profile, if the quality of links is low or certain percentages between follow/ nofollow links are different then it could be a sign.
- Look at the site with Google as a user agent and see if the site is showing different information then to a real user.
- Just do a select all on the site to see if they hide any content (yup, still happens).