Google defines Adult Content for AdSense

While it’s fairly easy for Google AdSense bot to identify pages for blacklisted words (Stop Words), it’s much harder to figure out if the contents are all made up of images or photos. This is particularly true for questionable content such as adult or mature materials.

The Inside Google Blog has somewhat defined what is adult or mature content.

Our policy regarding adult or mature content may include any material that is not appropriate for all audiences. While this obviously includes full nudity or sexual activity, it may also include textually explicit sexual content, image or video content containing lewd or provocative poses, strategically covered nudity, see-through or sheer clothing, and close-ups of anatomy that would be inappropriate if shown nude. Additionally, topics such as sexual health and sex tips may be held to a higher standard of professionalism than content that isn’t bordering on mature.

And the litmus test?

… we sometimes suggest that publishers ask themselves the following questions to determine if content is family-safe: Would I be comfortable viewing this content with my parents or children in the same room? Would I feel comfortable viewing this content if my boss walked up behind me while I had this content on my screen? If the answer to either of these questions is no, then it is likely that some advertisers would not be comfortable showing their ads on such content and we might consider it mature.

For AdSense Publishers with a good record history, the AdSense team might warn you on the first instance (received a warning before for such violation for Motorcycle Philippines though the mistake was just a plain oversight). Any succeeding offense might result to account suspension. It becomes obvious now that Google manually checks photo-heavy websites for such type of content. Users can also use the Google Feedback to report such violations.