The bases of the metatarsals and the tarsal bones are the most reliable rotation indicator on the DP view.
If the foot is over rotated externally, the metatarsal bases will be heavily superimposed whilst the tuberosity of the navicular bone can be seen in profile.
Over rotation internally will open up the metatarsal bases and the resultant image will bear close resemblance to the medial oblique projection.
In the scenario where the DP is ordered to query a foreign body, do not angle the x-ray beam to mimic the arch as this will result in elongation of the foreign body in question.
In trauma, the patient may not be able to flex the affected knee to the desired angle. In this case, a triangular wedge can be placed under the foot. The same principle of angling posteriorly to mimic arch applies. With the foot resting at 10-15° less of an angle is required.