Do you mean, do they believe in God to the extent of being saved, or, do they believe that there is a God of some kind?
In the second case, only they can answer that question. And, even devils know there is a God to which they are ultimately accountable. But that is not enough to grant them eternal life with Messiah Jesus.
In the first case, the question is do they accept (believe) that God loves them so much that He allowed His one and only human born Son to die in their place, and, that three days later God raised Him from the dead? (According to the Bible, this is “believing in Jesus” and only test for salvation.)
One can argue Christian doctrine through torture and death, but that is not what Jesus wants from us.
Mt 7:21-23 "Not everyone who says to me, `Lord, Lord,' will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. Many will say to me on that day, `Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?' Then I will tell them plainly, `I never knew you. Away from me, you evildoers!'
Jn 5:22-24 Moreover, the Father judges no one, but has entrusted all judgment to the Son, that all may honor (love) the Son just as they honor (love) the Father. He who does not honor the Son does not honor the Father, who sent him. "I tell you the truth, whoever hears my word and believes him who sent me has eternal life and will not be condemned; he has crossed over from death to life.
Jn 13:34-35 "A new command I give you: Love one another. As I have loved you, so you must love one another. By this all men will know that you are my disciples, if you love one another."
Jn 14:21-24 Whoever has my commands and obeys them, he is the one who loves me. He who loves me will be loved by my Father, and I too will love him and show myself to him." Then Judas (not Judas Iscariot) said, "But, Lord, why do you intend to show yourself to us and not to the world?" Jesus replied, "If anyone loves me, he will obey my teaching. My Father will love him, and we will come to him and make our home with him. He who does not love me will not obey my teaching. These words you hear are not my own; they belong to the Father who sent me.
Jn 16:27 No, the Father himself loves you because you have loved me and have believed that I came from God.
I am convinced the most important question God asks of us is not whether or not we believe that He exists, but do we accept that He loves us beyond our imagining. Can “I” accept that He loves “me” so much that He allowed His one and only human born Son to die in “my” place, and, that three day later God raised Him from the dead?
I demonstrate my belief in His love for me by loving His Son, Messiah Jesus. The extent to which I live my life loving Messiah Jesus (and reflecting His love to others) is the extent to which I accept that He loves me that much.
For me, this is what “truly believing in God” means.