What is the ruling on the fast of one who doubts the emission of semen after looking at women in Ramadan; does he owe a day in compensation (qada') or expiation (kaffarah)?

Doubt concerning the emission of semen or the occurrence of anything that invalidates the fast does not invalidate it, because the default assumption is that the fast is valid. As for the emission of semen due to looking, there are cases that have been detailed previously. It is necessary to point out the impermissibility of looking at a non-mahram (definition) woman without necessity, and the sin is amplified in Ramadan (definition).