Here’s a good article that details exactly what sincerity to Allah is and how one can actually achieve it.
PS. Be sure to read the question fully. It’s quite interesting.
The meaning of being sincere towards Allah and how the Muslim can achieve that
Worship has three pillars, one of which is being sincere towards Allah. How can we be sincere towards Allah? I am very ambitious, and I want to acquire the greatest amount of shar’i knowledge, and I want to be a daa’iyah. Allah has blessed me with many talents which are needed for da’wah and spreading knowledge, and I want to be strong enough to pray qiyaam and fast. I have memorised the Book of Allah in full and now I am in the process of reinforcing my memorisation and learning how to recite it properly. I am learning shar’i knowledge and I have a number of projects and ideas for serving the Book of Allah and supporting the Deen, but my progress is slow for a number of reasons and because of many obstacles, especially from my family and the society around me — how much I am suffering from them! Because of physical weakness and health problems, I am not able to expend a great deal of effort, and if I do, I will spend several days unable to move. I always write out schedules for all these tasks and I try to follow them, but I cannot do that because of changing circumstances. I live on my own and I cannot find a sister to share my goals or a leader to help me and follow up with me so that I can fulfil my ambitions. I have looked a great deal but I cannot find anyone and I do not know the reason why I am so slow; is it because sincerity towards Allah is something that I have not achieved yet or what? This makes me very sad and pains me a great deal, and holds me back from reaching my goals. If there is any advice, then do not withhold it from me, because I am in the greatest need.
Praise be to Allaah.
Sincerity towards Allah is the best type of sincerity. The Muslim is being sincere towards his Lord if he achieves sincerity in three aspects: faith and proper belief, acts of obedience, and morals and manners. Faith does not mean mere wishful thinking. The one who is sincere in his faith is the one who attains faith in the way his Lord wants from him, which includes sincerity in faith, sincerity in intention, and sincerity in fear of Allah, may He be exalted. Not every act of obedience is sincere unless it is done, both outwardly and inwardly, in a manner which Allah likes.