Been on the road the past week and haven't been able to answer this the way I'd like. Here's a bit of a shotgun answer tho.
The Holy Spirit isn't something we can truly quantify. We are called to test it against the Scriptures tho. A good example would be "speaking in tongues" we see currently in charismatic churches. They claim they are indwelled with the spirit and are speaking in a heavenly language. If we view how tongues are spoken in church and what the Scriptures tell us about this practice we find it unbiblical. So no matter how it "feels", we can conclude that this example of being spirit filled is not from God. We need to do this each time we feel the Holy Spirit leading us somewhere or to something.
If God calls us to do something that goes against clear Biblical teaching, we can conclude it is not of God. An example: Your parents want you to go to college but you feel God is calling you to be a missionary to some country? Obviously, since God tells us to honor our parents by obeying them, this is probably not the leading of the Holy Spirit.
Interpreting Scripture thru the help of the Holy Spirit goes the same way. If God teaches us something in one part of the Bible that contradicts something in another part of the Bible, we know the spirit leading us is not of God.
So basically, every time one exercises discernment it must reflect back on and be viewed thru the lens of Scripture. As we build precept upon precept be build our spiritual muscles and are better able to eat the meat instead of just drinking the milk.
Fundamentals of the Faith are very important and should be clearly defined. But what do you propose we do with those who's viewpoints fall outside of ours? Who use other sources besides just Scripture to define those fundamentals? What about those who use the same words we use but define them differently?
Christ is the most central part of Christianity yet Mormons have a different view of who Christ is than most other Christians, Muslims believe in Christ yet have a different view of who He was. Even Catholics have a different view of Christ (viewed thru the lens of the Virgin Mary) than other Christians. Because we don't agree with them on "Who Christ is", do you feel we should exclude them from this forum, these conversations or our fellowship? Or should we know what we know and then agree to disagree?