How do we know the effectiveness of prayer?

James 5:16 states that the effectual fervent prayer of a righteous man avails much. However, this effectiveness is not attributed to our own merits but to the righteousness of Christ. Believers understand that their prayers are heard and answered based on Christ's completed work, not due to their own righteousness. In John 17, we see Christ praying for His followers, illustrating how His prayers hold divine power. Therefore, the assurance of answered prayer is rooted in the believer's union with Christ, who intercedes as our righteous advocate before the Father.