What is the ruling on the dampness that affects underwear after using the restroom, and is it considered najis if there is no trace of liquid?

Worrying thoughts (Waswasah) are evil, and you must fight them off and not succumb to them. You are not required to wash any part of your body or clothing unless you are certain it has been contaminated with impurity. If you doubt the emission of madhy (pre-seminal fluid), the default is that it has not been emitted. If you are certain it has been emitted but doubt whether it has touched your clothes, the default is that it has not touched them. Build upon this default and act according to it until there is absolute certainty to the contrary. The same applies to moisture: as long as you are not absolutely certain that it is an impurity that has exited you, nothing is required of you concerning it.