Viewing Hidden Content

Hello everyone!

Few days ago I met a guy who knew what was in "hidden content" without replying...I tried to figure out what he did but couldn't...Tried many tutorials on google and just couldn't do it. So my question is ,does anyone here know how'd he do it? It's really amazing and he done it in a few minutes!

Any help would be appreciated.

Bump , i would pay for one that works .