(15) See then that ye walk circumspectly, not as fools, but as wise,
(16) Redeeming the time, because the days are evil.
(17) Wherefore be ye not unwise, but understanding what the will of the Lord is.
We already understand that we must walk wisely and redeem the time. Beyond that, though, we are to understand the will of the Lord - what it is. How do we know God's will? We must redeem the time and instead of seeking after this world's ideas, seek first God and His kingdom.
Mark 3:35 - For whosoever shall do the will of God, the same is my brother, and my sister, and mother.
Doing the will of God is evidence that we are one with Christ.
Acts 13:36- For David, after he had served his own generation by the will of God, fell on sleep, and was laid unto his fathers, and saw corruption:
God has a specific will for specific children. For David, God's will was that he would serve his generation by being king.
Acts 21:14 - And when he would not be persuaded, we ceased, saying, The will of the Lord be done.
As believers, we should be surrendered to God's will for our life and the lives of others.
Romans 1:9-10 - For God is my witness, whom I serve with my spirit in the gospel of his Son, that without ceasing I make mention of you always in my prayers; Making request, if by any means now at length I might have a prosperous journey by the will of God to come unto you.
Romans 15:30, 32 - . . . strive together with me in your prayers to God for me; That I may come unto you with joy by the will of God, and may with you be refreshed.
In our prayers, we can make our requests (and even ask others to join in this request), but must be submitted to the will of God.
Romans 8:27 - And he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to the will of God.
While we pray, Jesus Christ (our Intercessor) intercedes on our behalf, but submitted to the will of God.
Romans 12:1-2 - I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
To prove God's will in our lives, we must first be renewed in the spirit of our minds - renewed according to God's way, not to the way of the world or our sinful flesh (the "old man"). God's will is good, acceptable, and perfect. Part of proving God's will in our lives seems connected with the first verse: presenting our bodies as living sacrifices, living holily, and doing what is acceptable in God's eyes. Perhaps to know God's perfect will, to be renewed in the spirit of our mind, we must apply those three aspects.