How can one tell that it's fake? How can you tell the difference between fake and real cum? Is it like the difference between iron and irony?
Well, to answer your first two questions: A) It just looks different, more like the frosting on a cinnamon roll than actual cum, B) There's often way more of it than is humanly possible (see the fake pics above), C) It comes out of a dildo instead of a dick.
To answer your third question: No, not at all.