Form aside, that is a fair question. How is one supposed to know whether the Holy Spirit is indeed acting, or if one is simply deluded or self-deluded?
Fair, but the implication is that there ought to be some easy rubric by which we can make this discernment. There's not. But it involves analyzing the results of the actions putatively produced by that so-called spirit. The actions of the Holy Spirit lead to peace, love, joy, faithfulness, self-control, patience, kindness, goodness, gentleness, and so forth. It may or may not be attended by flashy displays of miraculous powers -- usually not. It's also a matter of comparison of the results with what scripture says are the typical actions of the Holy Spirit.
Of course, that opens the Christian up to the charge of circularity. We know it's the Holy Spirit in case A because scripture tells us that the Holy Spirit behaves as in case A. Well, how do you know that the bible is trustworthy? Well, it's because it has been given us by God, the Holy Spirit. Is there a way to break the circle?
Yes. We can treat the bible as a book of trustworthy history (of course, arguments can be marshalled for this, but I don't need to do so for the present thread). That book explains how Jesus is the Jewish Messiah and therefore Lord of the whole world. He promised that his departure from this world would usher the Holy Spirit into the world, particularly among the community he had formed. He said that the Holy Spirit would remind them of all truth concerning Jesus and lead them into all other truth. As a result of this coming of the Holy Spirit to the primitive church, some of the members, particularly apostles and their first converts, wrote books by the power of that Holy Spirit. Because they were so empowered, the books/letters they wrote are authoritative. It's those books and letters that tell us how to discern the Holy Spirit.
This argument may have myriad problems and needs for qualification and clarification, but at least it's not circular. And it shows that it's epistemologically sound to use the scriptures as a basis for discerning whether a phenomenon, effect, influence, or action has been a result of the Spirit's work.
There's a question about just how much information the scripture provides. I don't think that the scripture provides us enough information to discern things in all cases. So we have to appeal to our own familiarity with the Spirit in discerning the Spirit. It's imperfect and there are plenty of opportunities to get it wrong. No doubt we get it wrong sometimes. But that's no reason not to make judgments.
