What should one do if they wake up and find their trousers wet, and how can they differentiate if it is urine or something else, and is it permissible to pray in them if they dry and it was urine?

It is hardly hidden to distinguish urine from other things by its smell. If a person wakes up and finds wetness and knows that it is urine by its known characteristics, he must cleanse himself and purify his clothes if he wishes to pray in them. He may sleep in those clothes again if the impurity is dry and his body is likewise, because purifying clothes from impurity is only obligatory if one intends to pray in them. However, if he doubts whether it is urine or something else, then he has a choice between what he doubts.