Oh you mean 
rotational symmetry
Follow steps:
1) Identify centre of image.
2) Rotate it 
360 degrees, a COMPLETE turn. And see how many times the image fits in EXACTLY as you are rotating.
  Like for a square, the image will in perfectly 4 times. So, a square has rotational symmetry of order 4. 
360 degrees, and fit in PERFECTLY, and don't forget the centre.