How does God search the heart of the believer?
God searches the heart of the believer through the indwelling Holy Spirit. The Spirit, the third person of the Trinity, lives in the believer, and He makes intercession for the saints according to the will of God. This intercession involves all three persons: the Spirit intercedes to the Son, and the Son intercedes to the Father.
Les explains that the Holy Spirit is the third person of the Trinity and is in the believer, though this is a concept we cannot fully understand; it is taken by faith. It is through that indwelling Holy Spirit that God searches the believer's heart, because the Spirit makes intercession for the saints according to the will of God. This does not leave out the Son; all three are involved—the Holy Spirit intercedes on our behalf to the Son, and the Son intercedes on our behalf to the Father. Les also connects this to the believer's assurance that all things work together for good, not for everybody, but for the believer.
And he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to the will of God.