Eyes can pretty well tell the difference between continuous reality and framed video.
The threshold to where the brains can start fooling themselves to think they're looking at a moving picture instead of frame after frame is somewhere around 24 FPS, but the higher the FPS, the closer to continuous the image stream is, and the less obvious the frames become.
The biggest thing that reveals frame rate is fast panning motion or an object moving fast through the image. In reality, the image is continuous so the eyes and (more importantly) brains will most likely pick the object up, calculate the route of the object and create an illusion that the eye was tracking the object all the time. With framed video, it doesn't work quite as smoothly.
Aside from that, peripheral sight can pick up very fast movements and short light signals...