How do we differentiate between the truly religious person and the hypocrite?

One should think well of those whose outward appearance is religious, and it is not permissible to accuse their inner thoughts or to follow people and uncover their private matters, for Allah alone knows what is in hearts. The difference between a believer and a hypocrite is that a believer sees their sins as grave and repents from them, whereas a hypocrite does not care about them, as Ibn Mas'ud said: "Indeed, the believer sees his sins as if he is sitting under a mountain, fearing that it will fall upon him. And indeed, the hypocrite sees his sins as a fly that passed by his nose, and he brushed it away like this."