The Catechism in Questions & Answers

203. How can we recognise the activity of the Holy Spirit?

The activity of the Holy Spirit becomes clear in that God became human in Jesus Christ: the Holy Spirit came upon Mary (cf. Luke 1: 35), and she became pregnant.
We also recognise the activity of the Holy Spirit in that He grants human beings knowledge of divine truth (revelations and insights). Concerning this, Jesus said: “But the [...] Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all things that I said to you” (John 14: 26). It is in this manner that we today experience the activity of the Holy Spirit in the sermon, especially in that He keeps alive Jesus Christ’s promised return.
The Apostles fulfil their tasks because they are filled with the Holy Spirit. “And when He [Jesus] had said this, He breathed on them, and said to them, ‘Receive the Holy Spirit’” (John 20: 22).