‘You have proved my heart, you have visited me in the night,
You have tried me, and find nothing,
I am determined that my mouth shall not transgress.'
‘You have proved my heart.' That is, have tried it and tested it and found out the truth about it.
‘You have visited me in the night.' The night is the time when men can be alone and the truth can come out. It is at night that a man's thoughts roam freely and people consider mischief (Psalms 36:4). It is also possible that when seeking a solution in a case the judge would visit a man at night when, alone together in privacy, he may be able to discover the truth. Compare how Nicodemus came to Jesus by night in order to find the truth (John 3:1). But YHWH has visited him and tried him then and found nothing. Indeed he is determined that nothing that he says will suggest transgression against God's Law and against His requirements.