How can a Muslim be certain of the transfer of ritual impurity (najāsa) from underwear to trousers directly touching it, and what is the ruling concerning that?

Filth is not considered to have transferred to the trousers from the inner garment unless the presence of a characteristic of filth, such as color or odor, is confirmed therein. However, in cases of doubt, the default is the purity of the outer garment and that the filth has not transferred to it.