You only need to look at 2 pieces. If you put the one correctly oriented corner in the bottom left position (or the corner and 2 edge block for 3x3) and look at the corner to the right of it you can tell which you have. If the top colour part is pointing towards you, you have sune, if it is on the right, it is anti-sune.
You can also look at any 2 unoriented corners, if they seem to be going in an anti-clockwise direction (to the right) it is anti-sune. If they are going in a clockwise direction (to the left) it is sune.