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Quote from IXIArblargIXI » What does this have to do with General off topic?
http://www.minecraftwiki.net/wiki/Endermen also this
http://www.minecraftwiki.net/wiki/Mobs#Confirmed_Future_Mobs (to see a better look at their eyes).
This ^
Anyways, they change color when you look at them to red; meaning they're hostile.