How Did We Know That The Resurrection Was A True Event

How did we know that the resurrection was a true event?

He was buried, and he was raised from the dead on the third day, just as the Scriptures said. (1 Corinthians 15:4 NLT)
The Bible said so, in prophecies before the event, and also when the event happened in fulfilment of the prophecies.

And if Christ has not been raised, your faith is futile and you are still in your sins. Then those also who have fallen asleep in Christ have perished. If in Christ we have hope in this life only, we are of all people most to be pitied. (1 Corinthians 15:17-19 ESV)
The next most important evidence that resurrection is a true event is the existence of Christianity. Unlike many religions, Christianity is not started by one man's ideas. Christianity is started by a group of people who witnessed an event - the resurrection of Jesus Christ. They were convinced by His resurrection that He is the Lamb of God, offered as the Saviour of the world.

If the resurrection is true, what does it mean to you?

Jesus said to her, "I am the resurrection and the life. The one who believes in me, even if he dies, will live. (John 11:25 CSB)
It means that if you believe in Him, you shall have eternal life.