How do we know miracles are no longer necessary today?

The necessity for miracles has ceased as the complete canon of Scripture has arrived. In 1 Corinthians 13:10, Paul explains that when 'that which is perfect is come,' the gifts that were in part will be done away with. This 'perfect' refers to the full revelation of the gospel. The miracles served to authenticate the ministry of the apostles, but now we rely on the sufficiency of God's Word and the Holy Spirit for faith and salvation. This does not mean God cannot or does not intervene in the world, but that He uses the gospel and His Word as the primary means of grace.