How can I know if I committed the unforgivable sin?

from the old testament

• Israel repeatedly rejected God’s prophets and His guidance, showing that ongoing, informed rejection of God’s truth can harden hearts (e.g., Exodus 7–14; Numbers 14:11-23). Those who persist in rebellion ultimately face judgment, showing the danger of willful, knowing rejection.
• The Pharisees’ unforgivable sin involved consciously seeing God’s work yet attributing it to Satan. Similarly, the Old Testament repeatedly notes that those who knowingly reject God’s commands after understanding His truth bear greater guilt (Deuteronomy 17:2-7; 19:18-21).
• Even in Israel’s rebellion, God repeatedly called people to repentance through prophets (e.g., Isaiah 1:18; Jeremiah 3:12-14), showing that concern, conviction, and willingness to turn to God are signs a person has not committed a final, irreversible rejection.

from the new testament

• The “unforgivable sin” passage is found in Matthew 12:18–32 and Mark 3:22–30. Jesus had been performing various signs and miracles (Matthew 11:4–5; John 5:36; John 10:37–38). While the signs were clear indicators that He was from God (John 9:30–33), the Jewish leaders kept trying to entrap Him and discredit Him.
• When Jesus cast out a demon (Matthew 12:22), the Pharisees declared that Jesus was empowered by Satan (Matthew 12:24). Jesus pointed out the foolishness of their reasoning and ends the interaction with this: "Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come" (Matthew 12:31-32). Attributing the Holy Spirit's power to Satan is the unforgivable sin.
• From this point, Jesus began speaking in parables. The parables were His intentional means of relaying truth to those “with ears to hear” (Matthew 13:16), while also preventing hardened leaders from understanding them. He did this as a judgment against the Pharisees “lest they should see with their eyes and hear with their ears and understand with their heart and turn, and I would heal them” (Matthew 13:15b).
• What makes the Pharisees' sin unforgivable is they knew the miracles were legitimate, but even knowing that they attributed Christ's power to Satan. Jesus' raising of Lazarus is a perfect example of their hard-heartedness (John 11). The Pharisees didn't dispute that Jesus had raised Lazarus, but they were so determined to remove the threat to their religious influence (Jesus) that they sought to kill not only Jesus, but Lazarus, too (John 11:47-48, 53; 12:10). No one today can commit the unforgivable sin because we are not in the presence of Christ as they were.

implications for today

Sometimes our worries are, themselves, proof that we have nothing to worry about. A parent who worries about being a good parent is likely a good parent because they care enough to worry. Someone who worries about having committed the unforgiveable sin likely hasn't because they care enough about Christ to have that concern. Besides, no one today can commit what Jesus declared the unforgiveable sin because none of us is in Christ's presence witnessing His miracles as the Pharisees were.

But our hearts can become so hard that they will no longer repent. This is because sin hardens hearts (Hebrews 3:13). If you come to know more about Christ and still reject him, your heart will harden (Hebrews 6:4–6). While there’s not a “tripwire” sin, you may lose the capacity or desire to repent.

This does not mean that God can’t overcome a hardened heart, nor does it mean that we should stop praying for those who have constantly rejected Jesus all their lives, but it does mean we need to watch ourselves not to assume there will be another chance to repent and be saved later.