Being in the flesh means being led by the flesh. Likewise being in the spirit means being led by the Holy Spirit (Romans 8: 8-10). John was in the spirit on the Lord's day (Revelation 1: 9-10).
He was sensitive to the leading of the Holy Spirit.
Jesus had prophesied about these days when we would worship the Lord in truth and in spirit; that is worshipping God as the Holy Spirit leads us (John 4: 21 - 26). No one can serve the Lord Jesus Christ without having the Holy Spirit (1 Corinthians 12:3).
All believers can and often do walk in the Spirit and therefore pray in the Spirit simply because they have the Holy Spirit living in them (Romans 8:9). In practise, as long as you are not living in sin, are sincerely following the Lord, and are not praying out of spite or with evil motives, you are most likely to be praying in the spirit.
Of course, as we grow in the Lord our praying in the spirit will become deeper and more intense. The Holy Spirit desires to lead us into deeper things of God. He desires to pray through us not only in our own language but also with tongues and groanings (Romans 8: 26).
Praying in the spirit means praying as the Holy Spirit leads you. When you pray in the spirit with tongues you allow the Holy Spirit to pray through you and lead your prayer (Romans 8: 26 - 28). Likewise you can allow the Holy Spirit to lead you in prayer as you pray in your own language. It is the depth of your walk with the Lord that determines how much the Holy Spirit can lead you in your prayer life and walk with the Lord. If your walk with God is shallow so will your prayer life be shallow, irrespective of your speaking in tongues or not.