Your brain has all kinds of poorly understood cues to know the difference between real danger and no danger and reality and computer graphics. One I notice is in cgi there is often a lot of charcoal grey like shadowing. And reflections of the environment are still exceeding hard to get right. That helicopter body is reflecting all of the things around it. Its very hard for computers to crunch all that although they are getting better all the time. Now the easiest way to recognize cgi is when there is simply too much action.