This has been and is a most interesting thread. I think there are more honest questions and thoughtful answers to this question than I have seen in a long time. All of these give me time to pause and reflect on just who Holy Spirit is and what He does to us, through us, and for us.
I can remember years ago before knowing Jesus and having a personal relationship with Him, I lived a life of which I am not proud. It never bothered me to do things that now would never do now, and none of those things bothered me even a little bit. I didn`t think of myself as a "bad" person or a "good" person, I just didn`t think of anything but what I wanted and what it would do for me. Never did I consider that I was sinning or causing a problem for me or anyone else, it was just who I was...or "what" I was. Satan never bothered me in those days, he didn`t have to....I was his already and cruising along just as he would have me do. I joined the church, went to SS, and even taught SS on occasion. Still no problems. Nothing much changed for me.....until I had a big, huge, problem that seemed to come up from nowhere. No way around it, over it, or anyway to avoid it. That is when I realized that I knew who Jesus was but I didn`t have a relationship with Him.
Turning point, life changing turning point, stunning realization that I did not have what I thought I had...an actual relationship with the Living God through His Son. Shorten the story here...all that was changed then and there! So what difference did that change make? Well, back to the original question of this thread.
How do I know I have Holy Spirit in me? Short answer is, I know right from wrong before I even do it. Years ago it didn`t bother me....now it does. An unkind word, a short answer, unkind thought, and I know it before the words pass my nose. Do I still slip up and do wrong things? Yes, but the request for forgiveness and repentance is immediate. How does all this take place? I have a personal counselor, teacher, and comforter who lives in and with me 24/7. Bible study? Not a problem, I get to read with the One who had it all written. There is a peace, a joy, a fullness of life, and total lack of worry that comes with this relationship with the God of the universe, and I must attribute it to having His Spirit living with me and in me all the time. Not just when I THINK I might need Him. Where exactly in my body does His Spirit live? I don`t know exactly, but why should I care? He is there and that is what is important. Heart, brain, or mind, the main most thing is that He is there and we have a relationship that is eternal. He knows my name.....