Can anyone (even Christians) see someone's future? Can we trust such people? Does the Bible say something about this?
It is clear from the Bible that people themselves DO NOT have the ability to see the future, for themselves or anyone else. Only God can know what lies ahead and the only way we can know it is if the He reveals it to us.
There is such a thing as the gift of prophecy, and sometimes those so gifted may have something revealed to them from the Lord about what is to come — whether a future event or a future action by someone or a group of people. There are many examples in the Bible of God allowing His prophets to know what would come to pass in the days ahead. But those insights were given so that the prophet could sound a warning to individuals or to an entire nation. Let me repeat, prophets themselves CANNOT know the future without God revealing it to them. They have no crystal ball or power within themselves to see what is to come.
Along with those who have a genuine prophetic gifting, the Bible also warns about false prophets. Jesus said, “Beware of false prophets, who come to you in sheep’s clothing but inwardly are ravenous wolves." (Matthew 7:15), and the Apostle John encouraged us in a similar way, saying, "Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world." (1 John 4:1)
Notice John tells us through the Holy Spirit to "test the spirits" — this means to test any prophetic messages we may receive. We test them by comparing what they are saying to Scripture, and also by allowing the test of time to play out to see if what they are saying comes to pass. And finally, we test the prophet's own personal "fruit" — meaning their way of life. Jesus said, "You will recognize them by their fruits." (Matthew 7:16) Anyone with a true prophetic gifting will be humble and welcome this kind of testing. False prophets, on the other hand, will demand that you accept their words at face value and become annoyed if you insist on putting them to the test.
If you are currently in an environment where people are often claiming to know the future life and actions of others, I would encourage you to be VERY cautious and perhaps even find a new fellowship. Although prophetic giftings are biblical and real, false prophets are just as real.