The Holy Spirit

If Christianity is true, it would seem that the best evidence available to us in the 21st-century is the holy spirit. In all my years as a Christian I never understood completely what this was exactly. I've read the Bible and I've listened to a lot of sermons so here is my feeble understanding...... the holy spirit is God's way of having an influence on the thoughts inside of our head. So... not only do we have our own, unique spirit (soul) that thinks inside of us - but a Christian also has a second spirit (God's spirit) that generates thoughts as well. So for example, a Christian will say that the holy spirit pricked their conscience to do some good deed, or the holy spirit is giving them a guilt trip for something bad that they've done. But here's my simple question... how do you know whether a thought is from the holy spirit or from your own spirit? What if everything we think is just coming from our own mind? I'm also open to the possibility that I've never experienced the real holy spirit. If this is the case, I'd like to ask Christians who feel certain that they've experienced the holy spirit: what is it like? ... and how do you discern between your own thoughts and those that come from the holy spirit?