Verily it is We Who have revealed the Book to thee in Truth: so serve Allah offering Him sincere devotion (Surah 39:2). Sincerity is a believers mark.
إِنَّا أَنْزَلْنَا إِلَيْكَ الْكِتَابَ بِالْحَقِّ فَاعْبُدِ اللَّهَ مُخْلِصًا لَهُ الدِّينَ
Firstly, correcting one’s intention, and bearing it in mind when starting to do the deed, is one of the most important things that the Muslim should focus on, for the acceptance or rejection of his deeds is based on that, as is the soundness or corruption of the heart. The one who wants to form a sound intention for his deeds must pay attention to the motive that is prompting him to do that deed, and he should be keen to ensure that his motive is -
seeking the pleasure of Allah
obeying Him and
complying with His commands.
By doing that, his intention will be for Allah, may He be exalted. After that -
he must maintain this original motive for doing the deed,
that is sincerely for the sake of Allah, may He be exalted.
He should not be distracted from that whilst doing the deed, and
ensure that his heart and his intention do not change change, or become diverted to something other than Allah, or associate anything else with his intention.
Narrated `Umar: I heard the Prophet (ﷺ) saying, "The reward of deeds depends on the intentions, so whoever emigrated for the worldly benefits or to marry a woman, his emigration was for that for which he emigrated, but whoever emigrated for the Sake of Allah and His Apostle, his emigration is for Allah and His Apostle."
Reference : Sahih al-Bukhari 3898; Book 63, Hadith 124; Vol. 5, Book 58, Hadith 238
Al-Bukhaari (6499) and Muslim (2987) narrated that Jundub said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever talks about his good deeds (to show off), Allah will expose him (on the Day of Resurrection), and whoever makes a (hypocritical) display, Allah will make a display of him.”
Meaning: Whoever does a deed that is not sincere (for the sake of Allah), rather he wants people to see him and to hear about him, will be requited for that, for Allah will expose him and shame him, and will make known what he used to conceal.
Secondly, He can find out whether he is being sincere in his deed, and whether he is striving only for the sake of Allah, by paying attention to the following:
He should not do that deed so that other people can see him or hear about him.
He should not focus too much on people’s praise or criticism.
Concealing his good deeds and keeping them secret should be dearer to him than doing them openly.
He should not increase his good deeds and make them look good when people are around him.
He should always think of himself as falling short, and should not attribute any good deeds to himself, and he should realize that all blessings come from Allah, and were it not for Allah, may He be exalted, he would be doomed.
He should pray a great deal for forgiveness (istighfaar) after doing the deed, because he feels that he has fallen short
He should rejoice when Allah enables him to do righteous deeds.
If a person pays attention to these matters with regard to his deeds, then there is the hope that he will be one of the sincere.
يٰۤـاَيُّهَا الَّذِيۡنَ اٰمَنُوۡا لَا تَتَّبِعُوۡا خُطُوٰتِ الشَّيۡطٰنِ ؕ وَمَنۡ يَّتَّبِعۡ خُطُوٰتِ الشَّيۡطٰنِ فَاِنَّهٗ يَاۡمُرُ بِالۡـفَحۡشَآءِ وَالۡمُنۡكَرِ ؕ وَلَوۡلَا فَضۡلُ اللّٰهِ عَلَيۡكُمۡ وَرَحۡمَتُهٗ مَا زَكٰى مِنۡكُمۡ مِّنۡ اَحَدٍ اَبَدًا وَّلٰـكِنَّ اللّٰهَ يُزَكِّىۡ مَنۡ يَّشَآءُ ؕ وَاللّٰهُ سَمِيۡعٌ عَلِيۡمٌ
Believers! Do not follow in Satan's footsteps. Let him who follows in Satan's footsteps (remember that) Satan bids people to indecency and evil. Were it not for Allah's Bounty and His Mercy unto you, not one of you would have ever attained purity. But Allah enables whomsoever He wills to attain purity. Allah is All- Hearing, All-Knowing. (Surah 24:21)
It should also be noted that the Shaytaan is keen to make the Muslim fall into one of two things:
either to make him do a good deed in order to show off and for the sake of his reputation, and not sincerely for the sake of Allah,
or to make him refrain from doing the good deed altogether.
The Muslim who is sincere in his intention will not pay any attention to the whispers that the Shaitan tries to impart to him concerning his action, or his suggestion that it is not for the sake of Allah. He will not pay attention to the whispers of the Shaitan suggesting that he should refrain from doing the good deed for fear of showing off. The sincere heart is equally at ease both when doing good deeds in secret and when doing good deeds openly.
As for being certain that one is sincere in one’s deeds, there is no way to verify that, because the knowledge of that is with Allah alone. But the individual should take measures to attain sincerity, and ask Allah, may He be exalted, to enable him to do his deeds in an appropriate and sincere manner, but he should not affirm that he or anyone else has attained sincerity.
Rest Allah knows best.
Source: Quran, Sunnah, IslamQA, Also visit: https://www.almohsinun.com/post/the-basic-rule-of-action-and-reward