If someone claims to believe in Jesus, ask them if they ever get hungry or thirsty. If they answer yes, that means they don’t actually believe in Jesus. If they answer no, you still need a little more evidence before you can conclude that they’re a believer.
Ask the alleged Christian if they obey everything Jesus commanded. Do they always give people whatever they ask for?1 Have they sold everything they owned and given the money away to the poor? Do they hate their families? Do they let criminals mistreat them as much as they want without resisting? Do they mutilate their bodies every time they do something wrong? Have they demonstrated their love for their friends by killing themselves? Anyone who claims to know Jesus but doesn’t do what he commands is a liar.
Next, ask them to show you some miracles like Jesus did. Jesus says everyone who believes in him can do all the same things he did, and more! So have them start off with some of the miracles that Jesus did, like mind reading, turning water into wine, turning a little food into a lot of food, walking on water, flying, disappearing, controlling the weather, curing every disease, and raising the dead.
Then you can have them move on to some miracles that Jesus never did, but that he will be happy to help his followers do, so everyone can see how great God is. Tell them to move a mountain! If they have any faith at all, that should be no problem.
Finally, anyone who believes in Jesus should be able to handle snakes and drink poison without being harmed. I don’t recommend challenging anyone to do those things until after they’ve undeniably passed all the other tests, though.2
According to Jesus himself, anyone who fails to demonstrate these signs is not a true believer.