The major hallmark of the individual who genuinely believes in Jesus Christ is that he/she is born again (i.e. born of God):

Everyone who believes that Jesus is the Christ is born of God (1 John 5:1, NIV; emphasis mine)

And what does it mean to be born again? It means to take on the new divine nature, and to as if one has become a new person:

 This means that anyone who belongs to Christ has become a new person. The old life is gone; a new life has begun! (2 Corinthians 5:17, NLT)

There are three major hallmarks of those who truly believe in Jesus Christ, and who are born again:

1. They do not make a practice of sinning

We know that God’s children do not make a practice of sinning, for God’s Son holds them securely, and the evil one cannot touch them. (1 John 5:18, NLT)

2. They overcome the world (as opposed to being enamoured by it)

for everyone born of God overcomes the world. This is the victory that has overcome the world, even our faith. (1 John 5:4, NIV)

3. They have love for fellow believers

Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. (1 John 4:7, NIV)

Overall, although belief is intangible, it can be proven. It is like the wind — although it cannot be seen, it can be felt (and indeed, the Lord Jesus Christ used that analogy to explain those who had truly believed and been born again).