How do we know the disciples were not lying?

Roman historians from the first century (not part of the Bible) records the persecution early Christians had to endure like Matthew was slain with an axe, Mark died by cruelly dragged through the streets of the city, Luke was hung to death, John was tortured and then banished to the island of Patmos (Revelation 1:9), James (John’s brother) was beheaded (Acts 12:2), James (the less) was thrown from a pinnacle of the temple and stoned, Philip was hung up against a pillar and stoned, Bartholomew was flayed alive, Andrew was bound to a cross and left to die, Jude was shot to death with arrows, Matthias (who replaced Judas) was stoned and beheaded, Barnabas was stoned, Paul was beheaded, Thomas was speared, Peter was crucified upside down. What are the chances that all these men endured this for a story they made up?

Usually when people protect a lie, they can endure only a 3-4 weeks before giving up. These men proclaimed the truth for 40 years.

For a more detailed answer you might be interested in this article.