Deciduous molar teeth have a different anatomy than permanent molars. Deciduous molars are smaller in size, whiter, and their crowns are proportionately shorter compared to their roots. Deciduous molars have thinner enamel that are less mineralized; their occlusal surface (biting or chewing surface) is narrow bucco-lingually (measuring from the tongue to the cheek), the cusps and ridges are not as high and not as pointed, and the fossae (valleys) are less deep, and there are less fissures and grooves, compared to adult molars. Orthodontic braces do not bond so tenaciously to deciduous teeth because of their anatomy & enamel structure.
Deciduous molar roots are thinner and longer compared to those of adult teeth. The root diverge out from each other more, and start diverging much earlier, from a point closer to the crown, than in adult teeth. The dentist can also distinguish the left and right molars easily based on their anatomy.
See information on deciduous teeth in Wikipedia.