In this verse, Jesus demonstrates that our inner person, including our motivations, is more important than our actions. This is not meant to suggest that what we do is unimportant but rather that improper motives make otherwise good deeds appear selfish. Those who demonstrate righteousness in the manner Jesus teaches are indeed rewarded in heaven by the Father. However, a person who is trying to gain the praise and admiration of others is not rewarded for their behavior.
It is God who searches the heart. Only He is able to recognize when a non-believer's heart begins to trust in Christ for salvation. In addition, He is aware of the heart of the believer. For Christians, His rewards depend entirely on the motivation behind the work we perform. No reward will be received from God for actions that seem good and righteous but are carried out in the flesh.
The motives behind all of our actions and words need to be critically examined so that we do not fall into the temptation to carry out hypocritical actions just to impress God and others with our piety for their warm applause. The Holy Spirit can see whether our actions and attitudes are motivated by self-glorification or God's glory.

Dear Heavenly Father, only You can see the inside of us, not those who look at the outside. I ask that You keep me from the temptation to do hypocritical works of the flesh in order to win a person's approval. Please examine my heart, find out all my hidden motives and expose everything that offends You. I pray that from this day forward I will live and work to Your praise and glory. 
In Jesus's name I pray. Amen.