“And do not be conformed to this world, but be transformed by the
renewing of your mind, that you may prove what is that good and
acceptable and perfect will of God.”
Romans 12:2 

When you renew your mind daily in the Word of God and keep communication with Him through prayer, your mind will reflect His thoughts and not the world’s. If your thoughts do not live up to God’s word, then you know that is not His will!

You can prove what is God’s will by renewing your mind in His word. However, if you are not renewing your mind, you will think as the world thinks and you will not see God’s will. His will is manifested through His word and prayer. Both will match.