Unless DJI and PolarPro got together and tooled their thread patterns to index the same for every one and the filter manufacturing plant indexes the glass filter the same on every filter ring, having an indicator mark would just make the filters cost 3x or more. Not to mention, that max polarized zone changes with the sun's azimuth throughout the day.
Quick way to orient yourself is to use your fingers of a hand and make like a gun (just like you were a kid). Point your index finger towards the sun. Now if you rotate your hand around your index finger, the arc your thumb points to is your max polarization zone. Point your drone's camera in that direction and while looking at the video feed on your tablet, slowly rotate the filter and note the darkening of the sky or foliage or reduced refraction on water. Once you hit the max effect, if you keep rotating the filter, the effect will begin to diminish. Just like focusing an old camera lens. When you get the max effect, just give knock it down by a 1/4 turn if you plan on having a lot of sky in your shots.
