One of the signs of proper understanding is when a slave takes care of his iman (faith) and pays attention to what decreases it, and knows when it increases and when it decreases and recognizes the temptation of shaytan when he comes to him.

 
Ibn Al Qayyim (rahimahullah) [Sharh Nooniyah Ibn Al-Qayyim by 'Eesa, 2/140]