It's really similar to special effects in tv/movies: Whenever there's something new it's hard to tell, but as we get used to it and start seeing the patterns it becomes easier to tell. The older the special effects are the more obvious it is. Quite often you can just kind of tell if something was special effects or practical effects without being sure why. And there are always some really well done ones that slip past everyone, or odd lighting that makes it look fake (a thread a day or two ago on here about a legal case involving a photograph, some people in the comments thought it was a painting).