Is it necessary to verify the ritual purity of slippers designated for entering mosque restrooms, both on their upper and lower surfaces, or not?

The fundamental principle is that a Muslim should guard against impurities (najāsat) and promptly wash anything that has been defiled by them. However, the Lawgiver (Allah) has waived the ruling concerning the predominant impurity found on footwear used for relieving oneself, and has affirmed the ruling of its rare cleanness, as an expansion and mercy to His servants. Thus, one may pray with such footwear without washing it, and it is not obligatory to ascertain its impurity or purity. Nevertheless, it is recommended to inspect it before prayer or before entering the mosque.