How should I respond when I doubt if someone is really a Christian?

God’s Response: “John said to Jesus, ‘Teacher, we saw someone using your name to cast out demons, but we told him to stop because he wasn’t in our group.’ ‘Don’t stop him,’ Jesus said. ‘No one who performs a miracle in my name will soon be able to speak evil of me. Anyone who is not against us is for us. If anyone gives you even a cup of water because you belong to the Messiah, I tell you the truth, that person will surely be rewarded.’” – Mark 9:38-41

God’s Promise: “Everyone who calls on the name of the Lord will be saved.” – Romans 10-13 ─ from One Year Mini Daily Inspiration (OMF Literature Inc.)