How can one differentiate between types of whispers (waswas), such as whispers of showing off (riya') and actually being a show-off, and between whispers of purity and the invalidation of purity?

Fear of showing off (riyaa') is commendable, but it should not lead to obsessive doubts (waswasa) and abandoning worship. Rather, the servant should strive to achieve sincerity and supplicate to Allah Almighty to ward off the afflictions of actions from him. As for abundant doubts in acts of worship, these are obsessive doubts (waswasa), and the one afflicted by them should disregard them and pay no attention to them, because giving in to them leads to great evil, and disregarding them earns the pleasure of Allah Almighty.