All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Sincerity means acting according to what pleases Allah only and for the Sake of Allah, and getting closer to Him while avoiding any other purpose such as showing off one's actions or seeking reputation, etc.
So, check yourself, if you feel that you are in the aforementioned situation, then praise Allah, otherwise, you have to spare no efforts to purify your intention. Moreover, do not allow devilish insinuations to get into your heart. If they strike your heart though you have purified your intention, then you should ignore them and should not give any attention to them. If you resist them, they will not harm you, but making efforts to get rid of them is a good sign. Imaam Muslim reported that some people from amongst the Companions of the Prophet came to him and said: Verily we perceive in our minds that which every one of us considers too grave to express. He (the Prophet) said: "Do you really perceive it?" They said: Yes. Upon this he remarked: "That is the faith manifest.”
Allah knows best.