A man’s mind or spirit may be known by his words and conduct; and so we may know God’s mind or Spirit by his words and dealings. The Bible teaches that those who come to God (by faith, and reformation from bad works and dead works, through Jesus) are accepted. (Hebrews 7:25) The questions a person should ask himself if he is looking for a witness of the spirit of sonship are:

Was I ever drawn to Christ? – Do I recognize Jesus as my Redeemer? Do I believe that through Jesus’s righteousness alone I have access to the heavenly Father and am acceptable with him? 

If you can answer yes to this question, the next question would be:

Did I ever fully consecrate myself – my life, my time, my talents, my influence, my all – to God? If this question also can be answered as yes, that person may rest fully assured that he has been accepted with the Father, in the Beloved One. God recognizes him as a son. 

Then in the present, a person should carefully look at his own heart’s desires. Is he still trusting in the merit of Jesus, and still consecrated to do the Lord’s will? Then he may allow the sweet confidence and peace which this thought of harmony with God, to fully possess his heart. 

This certainty of the Lord’s grace toward us in Christ is built upon facts of our own experience. It is built upon the unalterable character and Word of God. It is not changeable, as it would be if built upon the shifting sands of feelings. 

If doubts or fears intrude in some dark hour, we have only to take the “Lamp” (God’s Word) and re-examine the facts and the foundation. If our hearts are still loyal to the Lord, faith, joy and peace will instantly return to us. If we find our faith in “the precious blood” crumbling, or our consecration slipping away, we know the true condition of affairs. We can at once make the proper repairs and thus re-establish our “full assurance of faith.” (Hebrews 10:22) 

Each one who would have this assurance must believe “that God is true.” John 3:33 (NKJV). Our Lord Jesus changes not, but is “the same yesterday, today and forever,” Hebrews 13:8 (NKJV). The Lord’s people may be certain that once they come into the conditions of divine favor, they may continue under those conditions so long as their hearts are loyal to God. So long as they are at heart obedient to the divine commands – briefly comprehended in the word – Love – to God and men, they are sons of God. Hebrews 11:6; 13:8

During the present time, a son of God is specifically called to run to become a saint, one of the 144,000. (Philippians 3:14)

Additional Resources: 
Christian Questions Podcast 
Episode 1248: “What Will We Be Doing When We Get to Heaven? (Part I)”
Examining the privilege of heaven, why it is open and how to get there
Preview Video  
CQ Rewind Show Notes

Christian Questions Blog: Who is part of the 144,000 in Revelation?