FINDING THE NO-PARALLAX POINT
Hold up a pen in front of you, close one eye, and focus your attention on the background scene as you move your head sideways from left to right. The pen will be seen to move to the left, relative to the background. Like this: This apparent change in position of the near pen is an effect called parallax. Just as the pupil of your eye is the centre of the world it sees, so it is that the entrance pupil of the camera lens is the centre of the scene it sees. NB. Failure to rotate the camera about the entrance pupil is very likely to result in visible stitching errors. The bottom line is that if you want to check if your camera is correctly set up to avoid parallax, then you should do some tests designed specifically to reveal the presence of parallax.
Related: Tips and tricks