That's always a tough one isn't it? Specially when it comes to trying to explain it, cus "you'll just know" sounds so silly.

I think the closer your relationship with God is, the better you will be at hearing from him. Most of the time when you hear from God you can respond immediately with little consequence. If God tells you to tell someone something, it's easy to just tell them. If he tells you to leave an extra large tip for the waiter or waitress, it's usually safe to go ahead and do it. If he's telling you to stop to help someone, then by all means stop and help them. If he's telling you that this week he wants double the tithes, then do it. When it becomes more challenging is when you feel God is leading you towards something that has big implications. Is it really God telling you to quit your job to go into ministry full time? Is it really God telling you to seek a certain relationship? Is it really God telling you to sell your house and move to that place you've always had a heart for? Is it really God telling you to ask that woman to marry you?

I think there are a few things we can do to test such things.

1. Consult the bible. Is what you are being asked in line with what the bible says? If it fails the bible test, then it is simply not from God.

2. Ask your peers; those who know you well and share your beliefs. Chances are that if God is asking you to do something that completely makes no sense for you, your friends will be able to tell you. This is not a foolproof method, Moses would have almost certainly failed it, but even if they are wrong, getting their perspective will at least help you be aware of the difficulties you will face if you follow through on what you believe God is telling you.

3. Ask someone in your church leadership. When it makes sense to do so, specially if it something having to do with ministry or marriage for example, ask your pastor or another church leader who at least knows you. They may not know you personally as well as your peers might, but they probably have a lot more experience talking with people who say they have been led by God to do something and they have probably gotten good at spotting the warning signs in those who really didn't hear from God and noticing the positive signs in those who did. Also, they might just happen to have a gift of discernment which helps them tell if something is from God.

I realize that this violates one of the things Rick Warren suggests in the book (do it no matter how little sense it makes). But I believe if God is leading you to do something big, you will eventually reach a point where you can no longer go on NOT doing it. At that point, it won't matter what your peers or leadership said, you will know you just can't go on living your life without doing that. At that point, it will be clear it was from God because you just won't be able to get it out of your head, God will not leave you alone until you do it, just ask Jonah and you'll see.