Examine me, O YHWH, and prove me,
Try my heart and my mind.
He is so confident of his current faithfulness to YHWH that he is ready to open himself for a thorough examination. ‘Examine me.' He wants Him to test his integrity like an assayer tests the purity of metal. Compare Jeremiah 6:27 where Jeremiah was to act as such an assayer, ‘I have made you an assayer and a tester among my people that you may know and try their way'. See also Psalms 66:10. ‘Prove me.' By weighing him up, measuring him against the Law, and coming to a fair conclusion. ‘Try my heart and my mind.' By assessing all his inner thoughts. The word for ‘heart' is often translated kidneys, which were seen as the seat of the emotions, or as ‘reins', that which controlled those emotions (compare Psalms 7:9). The word for ‘mind' signifies that which controls the thought and will.
We should note that he is not asking God to send him trials in order to test him. He has had enough of those. Rather he wants Him now to examine the results of those trials in order to discover that they have accomplished their purpose (compare Psalms 11:4-5). We should never pray for trials to come on us, for as Jesus stressed, our prayer should be, ‘do not bring us into trials but deliver us from evil' (Matthew 6:13).
A prayer like this in public would rightly have been dismissed as showmanship (compare Luke 18:11-12). But in private it is the sign of a genuine desire to be pleasing to God. We should all be making such a prayer on a regular basis so that God can carry out His regular ‘service' on our lives.