A lower P2 is not hard to distinguish from a lower M3 (Fig.1). Actually these differences are caricatures of differences between other lower premolars and molars. About seventy years ago Gromova noted them: develoment of the hypostylid (short, wide and flattened on premolars, elongated and narrow on molars), relative anterior and posterior occlusal widths (Fig.2).
A third difference is in the angle between occlusal surface and dental crown: on premolars the occlusal surface is close to perpendicular to the crown while it is slanting on molars.

No distinction is ever one hundred percent sure but I have found Gromova’s observations correct and very reliable.