You can hold it up to your eye pointed at the general direction you are going to be shooting. Rotate and look for a darkening of the sky. Maximum effect is at 90 degrees to the light source (sun).Actually what I was looking for! You answered most of my questions. Now, to get out and do some flying and experimenting to get comfortable with it. One more question though, can you find the sweet spot of the PL by just rotating it right in front of the lens or would I have to actually fit it to to the lens?
Be aware that when the angle changes relative to the light source it will change the effect. This is most noticeable in some panoramas or at 45+ degree angles to the sun. One side will be deeper blue than the other. Also be aware the effect can change with gimbal angle.