You can quickly figure out if a PA is Mn vs Mx by locating the sinus.
Also, when you look at posterior xrays you can tell whether it's right side up by the upward curvature of the mandible.
Sometime the palatal root is difficult to discern. Compared side by side, a Mn molar will have two well-defined roots where a Mx molar looks more 'cloudy' between the roots.