I really like how people can guess a game engine from screenshots.
http://i52.tinypic.com/33kau0i.jpg
http://i51.photobucket.com/albums/f386/vtastek/morrowind/Morrowind2009/sc04/Morrowind-MGE-Crysis.jpg
http://level-design.org/wp-content/uploads/2010/08/level_haze2.jpg
http://level-design.org/wp-content/uploads/2010/08/level_deathrow2.jpg
http://level-design.org/wp-content/uploads/2010/08/level_dmworm2.jpg
You can recognize them from assets. Which shows a particular art design/style. But you can be mistaken because assets can be loaded with different engines or same artist may work on various engines. It is better to recognize them from engine capabilities but in this era, all engines have very similar capabilities and they can all fake a moment in a screenshot in one way or another. There are really few games which can do others cannot and that's always changing.
PS.
Spoiler All engine names are lies.
Spoiler Morrowind
Morrowind
Unreal
Source
Unreal