Well, you're not supposed to build sites specifically for AdSense. The wording in the AdSense terms and conditions is kind of loose, however, and doesn't really define what "made for AdSense" means. See What is an AdSense "content page"? and The AdSense content game for more information. (Those are blog entries of mine on this very topic.)
I used to think I knew what a Made For AdSense site was, but people can defend just about anything. Until Google boots them out of the program, they'll continue to toe the line. I took a look at a site (review here) that has keyword-driven content with very little useful information. I would call it a made for adsense site, but I'm sure the webmaster would get all defensive about it.
There may be very thin line between MFA and geniune site. But as torunforever said what may look MFA to one may not be so for other (owner).