How can a fasting person verify the emission of semen and not pre-seminal fluid, and what is the ruling on his fast if he finds opaque or white spots on his underwear after arousal without a wet dream?

If something is discharged during sleep, the fast is valid, whether it is mani (seminal fluid) or madhi (pre-seminal fluid). As for wakefulness, one can differentiate between mani (which is discharged forcefully and gushingly, is thick, and smells like dough) and madhi (which is clear, viscous, and odorless). If there is doubt whether it is mani or madhi, it is ruled to be madhi, and the fast remains valid.