Answers
Who is the Holy Spirit and how do I know He is in me?
Not a force but a Person — He teaches, grieves, decides and intercedes. Every believer receives Him at conversion; that is not a second-class category. The evidence that He is at work is less spectacular and more demanding than most people expect: love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control, growing where they were not growing before.
Go and see for yourself
But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
John 14:26
But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.
Romans 8:9
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
Galatians 5:22