The gospel comes to us through revelation. God's gospel isn't man's gospel; it is all of grace. The gospel is not in competition with the Scripture but according to Scripture.
How do we know?
The most significant proof of the gospel is the direction your confession takes you.
Does your confession take you towards self-reliance and self-justifying, or does your confession take you towards marvelous grace that says, my only comfort in life and death is that I am not my own? Do you boast in your good deeds, or is your boast in your Redeemer? An honest answer to those questions will tell you the road you are traveling.
As Paul learned, the gospel brings transformation. First on that list of transformation is a new direction.
Consider Paul, he was formerly a persecutor, now a missionary. He formerly was a pharisee, now an apostle. He was once advancing in the traditions of men, and now he understands the Bible.
The gospel gives us a new way. What is your new way? How has the gospel changed you? For some, the change is dramatic. In others, the change takes more time. All of us are being renewed day by day.
A disciple of Jesus is one who is irrevocably embarked on the Jesus way. (I am grateful for my missions prof in seminary, Ant Greenham, for this definition.) We have forsaken all others and are trusting in Him. We were headed towards self, sin, destruction, and despair, but are now, by grace, we are bound for the promised land of the glory of God.
Galatians 1:23-24 causes us ask the question, "Who is glorifying God because of you?
Paul's own life serves as proof of the change the gospel brings. He remembers the poor with eagerness. He turns from a zealous pursuit of self to considering others more significant than himself.
Look through your life and ask, “Who is glorifying God because of me?”