False prophets can be hugely dangerous but there are ways they can be spotted.
Firstly, check what they say against Scripture. Some years ago I came across teaching from a guy called Brent Engelman that I read and outright rejected. His teachings included things that were the exact opposite of what Scripture teaches. Where he taught "when the visions start accept they are from God", Scripture teaches "test all things, hold fast what is true". Be aware that many false prophets will use very find-sounding lines that lull people into a false sense of security. A classic one is the idea that "God will protect" and "God's power to protect is stronger than the devil's power to mislead". Which sounds very good, but it didn't work too well for Adam and Eve and won't work any better for us. God gave us Scripture as a yardstick, Jesus warned us of false prophets, so it doesn't work to just follow anyone and anything and figure God will protect us.
Secondly, check the fruits of their lives. Jesus commented how we would know people by their fruit, and how you don't gather figs from a thornbush. This is where we need to be careful with visiting speakers, or third-hand accounts of something that happened somewhere else, to someone else, but everyone marveled at how powerfully God moved. The guy who attends your church regularly is someone you can see and come to know. You can see how he treats his family, how his family act around him. You can see what he's like in the long line at the checkout, or how he handles slow wait staff, or being held up in traffic. You can see whether he is humble or full of himself. You can see how he presents himself, and how he presents anything he thinks God might be saying.
Check the delivery of the message. The person who boldly proclaims "Thus saith the LORD" is worth watching a lot more carefully than the person who quietly takes you to one side and says "Hey, I've been praying for you and think God is saying....". The person who encourages you to accept their words blindly is probably not a true prophet. The person who encourages you to test what they say and prayerfully consider it is more likely to have something of value.
Check the consistency of the message with other inputs you may have. This is another part of testing the message and will vary depending on how confident you are in the message and the messenger, and how much of an impact it may have on your life. To give you a couple of examples:
One time I was sitting in church, away from my wife (I was working the sound desk, we hadn't had a row or anything!) and we had a visiting speaker talk about a specific need his ministry had. The amount of money was enough that I'd think twice about just giving it to him but not so much that it wasn't an option. But I thought God was saying that we could see to it. I wanted confirmation and practically the moment my wife and I got home she asked me what I thought about meeting the need because she felt God was saying we should do it. The fact we both got the same impression at the same time despite being in different parts of the church was enough to satisfy us both that we should do it, so we did. But to reiterate, this didn't involve the kind of money that would really hurt us, so it's not like we turned our lives over on a hunch.
I have a situation which is ongoing so I'm going to be deliberately vague on the detail. This one is potentially a much more far-reaching change, so it took a lot more than a hunch to prompt us to start moving but, once my wife and I agreed that it looked like God was leading us and asked for clarification, all sorts of things both large and small started pointing the same way. Even then things were very tentative at first, pushing doors to see what would open and what would close. In a situation like this it's entirely possible God will close doors and say "No" but, this far at least, our answers have been a combination of "OK" and "Wait". With this one some of the earlier signs were arguably vague but as soon as I took things further they became clearer. For example, I spoke to our pastor one morning and the next day, in a church committee meeting, one of the other members presented something that made the pastor want to kick me under the table because it was so closely related to what we discussed the day before. Then when I took it outside the church and spoke to an external body to explore possibilities, I started coming under heavy spiritual attack so I figured that was a good sign I was doing the right thing. In a situation like this an unknown person saying to me personally "Hey, did you consider doing (stuff)?" that aligned perfectly with what I'm trying to do I'd regard it as a good sign. If, six months ago, some random person had said to me "God says you need to do (stuff)" I'd have laughed at him. If he'd taken me to one side and said "Hey, I really think God might be calling you to do (stuff)" I'd be inclined to consider it and, in the light of everything else, concluded it was one of many signs that God was using.
If a person says to a large group (as in the example of the visiting speaker, or the speaker at a rally talking to a huge crowd) that "there's someone here who feels a great calling and God is saying that you should stop doubting and follow that call" it's more or less useless. Statistically speaking in a large crowd there are probably many people who feel some kind of calling so the chances are that speaker is playing a numbers game. If people stand up they look good and the chances are sooner or later someone will think "hey, maybe that is me" and stand up even though they just have a vague idea that God might be leading them in a way that doesn't seem particularly great.
One problem with testing is that in many cases there's no clear Scripture to confirm or deny a calling you feel applies to you. For the sake of an example, if you think God is calling you to become a missionary to Bangkok there isn't a single verse anywhere in Scripture that will tell you whether that's a calling or your imagination. Jesus told us to "go into the world and preach the gospel" but "the world" means anything from the other side of the world to the village three miles up the road. If someone presenting themselves as a prophet tells you to go to Bangkok you should need a lot more than that before you put your house on the market. This is one of those awkward situations where you might find Scripture to cancel a message - to take a silly example if the "prophet" tells you to leave your spouse, or to do something clearly prohibited by Scripture you can safely ignore them - but you'll struggle to find Scripture to clearly support something specific.
If you don't have a chance to test the speaker, test the word. If you don't have chance to test the word, regard it with caution and seek confirmation before doing anything with it. If God is truly behind the words he knows what it will take to get you to move, and he's perfectly capable of providing as much confirmation as you need to press forward with it. If the "prophet" makes bold proclamations of what will happen and those things don't happen, ignore the speaker.