How can one verify whether the imagined drop of urine after Istinja’ (ablution after defecating or urinating) is a delusion or real, and is it considered a pardoned impurity if it is real?

"Whoever is certain of purity and doubts impurity, remains pure. Doubt, or a near approximation of it, is not given weight. It is recommended to sprinkle water on the private parts and trousers after ablution to ward off obsessive doubts. However, if one is certain of a drop of urine exiting, then the garment must be washed and ablution repeated. Certainty is not removed by doubt. So, unless there is evidence of impurity, such as color or smell, doubt is not to be considered. As for certain impurities like wady, urine, and feces, even a small amount is not excused."