Jesus words as recorded in Mark 16:16-18 identify those who hear the "good news" and become believers in this way: "By the use of my name they will expel demons, they will speak with tongues, and with their hands they will pick up serpents, and if they drink anything deadly it will not hurt them at all. They will lay their hands on sick persons, and these will become well."
I know a lot of people who claim to be Christians and true believers, but I don't think I'll ask any of them to prove it by the above criteria. That would just be too cruel.