How can one get rid of the obsessive doubt concerning the emission of semen, and is seeing it on clothes a measure for the necessity of ritual purification (ghusl)?

Whispers (Waswas) are from Satan, and a servant should repel them through remembrance (dhikr) and seeking forgiveness (istighfar). The criterion is seeing semen on the body or clothing. There is nothing upon a Muslim as long as he does not see anything, and he is not permitted to yield to these whispers.