The fruits and effects He produces
(J.C. Ryle, "The Holy Spirit")
"When He comes, He will convict the world about sin, righteousness, and judgment." (John 16:8)
Where the Holy Spirit is, there will always be deep conviction of sin — and true repentance for it. It is His special office to convict of sin.
He shows the exceeding holiness of God.
He teaches the exceeding corruption and infirmity of our nature.
He strips us of our blind self-righteousness.
He opens our eyes to our awful guilt, folly and danger.
He fills the heart with sorrow, contrition, and abhorrence for sin — as the abominable thing which God hates.
He who knows nothing of all this, and saunters carelessly through life, thoughtless about sin, and indifferent and unconcerned about his soul — is a dead man before God! He has not the Holy Spirit.
The presence of the Holy Spirit in a man's heart can only be known by the fruits and effects He produces. Mysterious and invisible to mortal eye as His operations are — they always lead to certain visible and tangible results.
Just as you know there is life in a tree by its sap, buds, leaves and fruits — just so you may know the Spirit to be in a man's heart by the influence He exercises over his thoughts, affections, opinions, habits, and life. I lay this down broadly and unhesitatingly. I see it clearly marked out in our Lord Jesus Christ's words, "Every tree is known by his own fruit." Luke 6:44