Well, John gives us a test.
1John 4:4 Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
King James Version (KJV)
So applying this test to all things should be enough to satisfy whether something is of God, especially if we can see examples of a thing in scripture. That is all I am saying.
There is a difference between a teaching and an experience. Moses had an experience, as he was not taught to expect what he encountered when God met with him. The best we can do is study scripture to know what others have experienced in God's Presence and prepare as they did, know what to expect, and prayerful test every spirit according to scripture.
These two posts combine to indicate a large part of the problem here.
If I have an experience and I am trying to figure out if it was something of God's doing I can compare it to Scripture to try and gauge it. If I believe God is leading me in a certain direction I can ask for specific confirmation and look for God to make it more or less clear, depending on how significant the leading is (for example if I feel led to give an extra $20 to a mission I won't need much confirmation; if I feel led to sell everything and become a missionary to Nigeria I expect God to make it very clear before I do anything). On that basis I can look at things on their own merits and make a decision whether I believe God is behind something.
The problem is when an experience becomes a teaching. With this whole "slain in the spirit" concept (as well as many other things that are ultimately experiences) we might look for a Scriptural precedent to gauge whether it is likely to be of God or not but that can only take us so far. The fact God never did something before doesn't mean he can't do it now, and the fact God did do something before doesn't mean he is doing it now. So while we might find from Scripture that something appears to align or conflict with the revealed nature of God we can't necessarily say definitively that someone is or is not of God based purely on whether or not God did it before.
To take an example, if we insist that God can't be behind something unless he did it before we would have to rule out the very existence of Jesus Christ the man because God never sent his son before. We'd have to rule out the act of creation because God never did that before. So if God cannot do something unless he did it previously we don't exist. The flipside is that God doing something before doesn't mean he is doing it now. God raised up apostles and prophets in the past but that alone isn't enough to support anything if I stood up and claimed to be an apostle. I can show that God raised apostles in the past but that doesn't prove I am an apostle
When an experience becomes a teaching we stray into all sorts of potential dangers. If you teach "slain in the spirit" based on your experiences I have no way of testing whether your experience was valid or not. All I can do is compare your teachings to Scripture in an attempt to determine whether or not it aligns with the revealed nature of God (to determine whether it is plausible), and from there the best I can do is look at the rest of your teaching and your fruit. If the rest of your teaching conflicts with Scripture then I'd be wary of anything you teach that relates to your own experiences; if your fruit doesn't look very Godly I won't put too much credence in your teachings. On the other hand if your teaching aligns with Scripture and your fruit are good I'll be more likely to accept your teaching. Since the phenomenon of "slain in the spirit" is something I've only ever seen in hypercharismatic churches that have assorted teachings I can't reconcile with Scripture however hard I try, I regard it with suspicion.
Of course on an internet forum I can't see you fruit and you can't see my fruit. Nobody reading this message knows what my prayer life is like, what my Christian walk is like, how I treat my wife and those around me, and so on. For all people reading this know I could be a humble and God-fearing man who treats his wife with Biblical respect and is kind and gentle with everyone around me; likewise I could be a drunken hooligan who beats his wife and has multiple affairs.
The test that John provides is an interesting one because comparing it to other passages it appears it must mean more than a first reading would suggest. For example when Jesus met demon-possessed men:
Mat 8:29 And suddenly they cried out, saying, "What have we to do with You, Jesus, You Son of God? Have You come here to torment us before the time?"
The demons recognised Jesus Christ who had come in the flesh but clearly were not of God.
