Great question, but the answer's not as clear as you would like. Sometimes they can be very difficult to tell apart. Let me explain.

First of all, Satan is very crafty and deceptive. 2 Cor. 11.14 tells us that he masquerades as an angel of light, so even the coming of an angel is no guarantee that the message is a genuine message from God. We know from Mt. 7.21-23 that even the working of miracles is no guarantee.

Secondly, we know that even sincere Christians can get knocked off track and pursue sinful objectives. Many people feel that the thoughts that come to their heads during prayer or Scripture reading are recognizable as "God thoughts", but I know from personal experience that is not necessarily true. Even some of the thoughts that come to mind during our times of devotion are planted there by the Enemy.

And, third, as I mentioned, we as humans are reliably terrible at interpreting our circumstances. In the OT, God often had to send prophets to interpret circumstances for the people. It was the word of the prophet that was reliable in telling us what the circumstances were about and how we could properly understand them with God's mind.

So is the fire alarm an innocuous event, an act of God, or a diversion of the devil? And how in the WORLD can we tell?

If you're looking for a clear cut way, you're fooling yourself. Life isn't so easy, and our Christian lives are not as clear-cut as we wish. But if you REALLY want to know how to tell, here's what I can offer you:

1. We need to check in multiple ways what we think are messages from God, or circumstances from God. Don't just assume.
2. The Holy Spirit gives the gift of discernment. Discern as wisely as you are able, and be wary of the easy answer or the answer you want to hear.
3. In prayer ask God for wisdom (James 1.5-6; Col. 1.9-10)—more than once. Wisdom is more often a lifestyle of growth and understanding than it is something God just dumps on you.
4. Seek the Lord in Scripture. Not every message that comes to us from its pages is an answer to our situation. But always seek the Lord in Scripture.
5. Do outside research where it's appropriate (Neh. 2.11-16; Lk. 14.28-32). God gave you a brain so you can use it.
6. Consult with other wise counselors (Prov. 11.14; 13.20; 15.22) who have spiritual insights and beneficial personal experiences.

Sometimes the fruit of an event can be tell-tale as well. A fire alarm went off. OK, were people truly led away from the Lord by that? Did anything happen that brought about a teachable moment where we saw the Holy Spirit at work? You know what I mean?

You may be thinking, "Are you kidding? I have to do all this every time something happens?" Not really. Wisdom, discernment, and maturity are more a lifelong lifestyle than a moment of enlightenment. We learn to walk in the ways of God, to see his hand, to be suspicious of our own thoughts and interpretations, and to discern truth. If you expect God to just dump insights and wisdom on you, that probably isn't going to happen. God is not so much a " dumper" as he is a discipler. He brings us along. We continually seek first the kingdom of God, learn how he works, and to recognize his patterns and styles in our lives. And sometimes we still don't know if it was God's hand, Satan's deceit, or simple happenstance. Often it may not matter, when it comes right down to it. You live your life in a walk with the Spirit, and whatever happens to you you learn from, seek to glorify God through it, and take a spiritual step with it.

I hope that helps. It may not have been the easy answer you were looking for, but I feel that it's an honest answer. Talk to me more if you wish.

I can see where people can misinterpret circumstance for God's will. With that being said, how does one truly know when it is God's will, and not just a random happening?

Let's take the fire alarm situation.
If the fire alarms were going off to disrupt the gathering for no reason what-so-ever, and was just a cause and effect
versus
The fire alarms went off to disrupt the teachings, so one soul didn't get saved to change the world
versus
The fire alarms went off disrupting the teachings, to bring about another kind of teaching moment, saving an individual, allowing them to change the world

As stated, I see that some situations deem no "God's or Satan's work", but how can you tell? Can we see God's plan? How can we know the purpose of happenstance, if any? Paul, was given a thorn in his flesh, and he could have left that as a circumstance. He took that as a way to not be conceited. I'm just curious, not trying to stir the pot by any means.

You want an honest answer from a Christian? Because people blab about things they don't understand, live comfortably in the sphere of cliches, and toss around interpretations of circumstances that come from their own heads rather than any places of authority. How does that strike you? Makes sense to me.

I have found through my experiences and exposure to Christians that they (not a whole lot different from the general population) are quick to create interpretations of circumstances and experiences. In attempts to find meaning and purpose in events, they jump to conclusions, say something shallow, create conversation, and move on.

Now, if you want me to be honest with you, which I presume you do, it's pretty simplistic and foolish. I'm not convinced that every event has a purpose. Sometimes things just happen as the result of simple cause-and-effect. Depending on what it is, sometimes we just blow through it, also assuming it's just cause-and-effect, and sometimes we do something with it and may start to give it a purpose. (Run a red light, get hit by another car, just cause-and-effect. Grieve, or learn, or create relationships with others as a result of said collision, and we start to invest the collision with purpose.)

Did Satan cause the fire alarm to go off? Ha. Hardly. Did God do it? Ha. Just as superficial. An alarm went off, that's all. No spiritual battle was won or lost over that one, I can guarantee you.

Are Satan and God at work in the world? I very strongly believe so, but not in foolishness. The stakes are real, and their moves are purposeful. But it's Mickey Mouse to attribute every circumstance to their work.

Sorry that you left the faith over silly reasons and misunderstandings of what God is really up to in the world. People should realize the damage they cause with such nonsense.

At what point does it change from Satan's warfare to God's will? A group of ladies from my former church were at a retreat this past weekend. The fire alarms apparently went off. Girl posts a picture and hashtags it as #satanswarfare. But if there ended up being a real fire and people died it would have been #godswill.