Hebrews 6:9-10 (ESV)
Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation.  For God is not so unjust as to overlook your work and the love that you showed for his sake in serving the saints, as you still do.
How did the author of Hebrews know that the people he was writing were not the false professors he has been talking about in the previous set of verses? Because, as verse 9 states, they had things that belong to salvation.
In verse 10 he mentions a few of these:
1) they worked – real salvation will always be accompanied by work; work that He prepared in advance for us to do (Eph. 2:10). The professing believer who is not actively serving Christ in exerting real effort, is not a Christian at all.
2) They demonstrated love in their work. They were not working to earn any favours from God. They were working because God, through their faith in Jesus Christ had planted a love for God and others that spurred them on to real effort.
3) They carried out these labours of love first of all, for the glory of God (“the love that you showed for his sake”). The real believer’s first love will always be the triune God. He is our first and deepest love. We may do great good for others out of a great love for people but it will never begin to approach the love we have for God. In fact, the love we have for God is what fuels the good we do for anyone else – all the time;
4) they worked in service to other believers. The church of Christ is a family and we love our brothers and sisters. Real believers will work for the benefit of others in the family, regardless of what else they do in the kingdom for the glory of God. This is how the world will know we are His disciples (John 13:35);
5) they persevere in the work they do, out of love, for the glory of God, and the good of the church. They do not say, “well I am glad that’s over and done with”. They know that they are called to serve in the way they do, until Jesus returns or until they die. Where there is no perseverance in the faith, there is no faith. The one who looks back is not fit for the kingdom of God. He who endures to the end shall be saved.
How about you? Do you have these bare essential elements of true saving faith? These are what God, the Holy Spirit produces when He indwells those who trust the living Christ to save them from sin. They will be the marks you have and demonstrate. And you will be glad that you can.