How does the believer know that his prayer (salat) and supplication (du'a) have been accepted by Allah, and what is the ruling on the lack of devotion (khushu') in prayer due to work circumstances or physical pain, and is he required to have the intention to wipe over the socks when putting them on after ablution?

The acceptance of acts of obedience is a matter of the unseen. A Muslim should strive to perform them in the prescribed manner and with sincerity. The righteous predecessors feared that their deeds might not be accepted. As for supplication (du'a), if it is for worldly matters, its acceptance can be known. And apart from sin and severing ties of kinship, supplication is answered. As for supplication for matters of the Hereafter, it is also a matter of the unseen. However, a Muslim is commanded to supplicate and is promised a response. He should have trust in Allah, think well of Him, and be steadfast. Humility (khushu') in prayer is its essence, and one should be keen on it. There is no sin in its loss if it is beyond one's will.