The teeth of pterosaurs are very difficult to identify in isolation. Their teeth shape can be very similar to the teeth of fish, crocodiles or small reptiles. In general, if a tooth is not associated with bones of the jaw, then it is unlikely to be a pterosaur tooth.
The earliest pterosaurs from the Triassic often had tricuspid teeth, with a large central cusp and two smaller lateral cusps. This form did not last beyond the Upper Triassic Era.
The evolution of pterosaurs is very directed along the lines of "lighter is better" in order to facilitate improvements in flying ability. In respect of this, the persisting tooth forms for all later pterosaurs are thin and curved single cusped teeth with hollow roots. The size and alignment of the teeth became modified in some forms, probably related to feeding needs, and often leading to a reduction in the number of teeth. In other pterosaurs, the teeth were completely lost.
Note: If you are buying an isolated tooth from a fossil dealer, it will carry a better price if it has the word Pterosaur associated with it. Buyer beware!