How can one differentiate between the types of wetness (wind, semen, pre-seminal fluid, urine, impurity) that appear on clothes, in order to overcome the obsessive doubts related to the presence of wetness in faded or transparent areas of clothing?

There is no difference in the obligation of ghusl (ritual bath) concerning the aforementioned matters, except for semen, according to the view that it is pure. So, if you see wetness coming out of one of the two private parts or on your inner garment, it must be washed, and you must perform Wudu (ablution). If the wetness recurs, you can refer to Fatwas No. 31631 and No. 37732.

However, it is necessary to verify that the wetness is indeed coming from the private parts or is impure, such as blood or pus. But if it is sweat, water, or if you are in doubt about it, then it is pure, and neither ghusl nor anything else is required. The default state of things is purity, and impurity is not ruled except with certainty. One must stay away from obsessive thoughts and doubts, as the Prophet (peace be upon him) said: "He should not leave (his prayer) until he hears a sound or finds a smell."