How do I ensure the sincerity of my intention to Allah, and what should I do when I experience the whisperings of showing off (riya') or the feeling of sincerity?

Sincerity (Ikhlas) is singling out Allah in intention during worship, meaning that a servant worships his Lord and intends none other than Allah in that worship. Rather, he performs acts of obedience seeking the countenance of Allah, the Most High, as He says: ﴿Say, "Indeed, I have been commanded to worship Allah, [being] sincere to Him in religion"﴾ [Az-Zumar: 11], and also His saying: ﴿And they were not commanded except to worship Allah, [being] sincere to Him in religion﴾ [Al-Bayyinah: 5].

In the Hadith Qudsi: "Allah the Almighty said: I am the Most Self-Sufficient, needing no partner. Whoever does a deed in which he associates another with Me, I leave him and his partnership." And the saying of the Prophet, peace and blessings be upon him: "Indeed, deeds are by intentions, and for every person is what he intended." So, whoever performs an act of worship seeking no benefit or praise from creation is, if Allah wills, sincere.

To overcome the whispers of Satan, it is recommended to continue performing righteous deeds while striving against oneself and correcting the intention, being cautious of what might corrupt it, and being vigilant of Satan's insidious approaches that might call one to abandon righteous deeds for fear of falling into showing off (riya'). Al-'Izz ibn 'Abd As-Salam said: "Satan calls to abandoning acts of obedience. If the servant overcomes him and intends to perform an act of obedience that is superior to others, Satan then presents him with showing off to corrupt it for him..."