Obeying the will of God is to obey His commandments. The will of God is His commandments. When we obey the commandments of God, we are doing His will. The purpose of God is expressed in His word. We know that the word of God is the commandments that God has given to us to obey. There is nothing else that we can do to be sure that we are doing the will of God. What we have to do so that we ourselves will know that we are in the will of God is to obey the commandments of God. If anyone presents another precept to us as the will of God, we should understand that the Word of God is the only true source document will contains the absolute will of God.
There is no other way to be happy in this world than living in obedience to the will of God. Those who are living in disobedience to will can never enjoy true peace. As the creatures of God, we were created to please God who created us. The will of God is for us to submit ourselves and obey God. King Saul missed this when he chose not to obey in totality the commandments of God. He foolishly obey the voices of the young men more than the voice of God. Because of that he was rejected. Those who are not obeying the commandments of God risk being rejected by God.
We are in the New Testament era. In this area, we do not follow the laws of Moses anymore. The doctrines of Jesus Christ to the church is what we have been given to follow now. As we follow the doctrines in the church of God as was given to us by the apostles who received same from the Lord Jesus Christ, we will find ourselves doing the will of God.
Matthew 28:19-20 (KJV) Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.