ashley86 asked a very good question. And one I had once asked myself before too, but never really knew the answer too. But here is what I believe the answer is: Speaking in tongues is of the Spirit. So I believe it must be revealed to us by God. Just as Jesus is revealed to us by God. You know when it is asked of us, how do you know Jesus is the Son of God. It is revealed to us.
And how do we recieve understanding of scriptures? By revelation.
There is also this scripture: try the spirits (1 John 4:1 Beloved believe not every spirit, but try the spirits whether they are from God: because many false prophets are gone out into the world)
So we can check out the truth, to see if it is really truth. Because that is what revelation is, truth. Having our eyes opened to the truth. And our minds opened to understanding (and we learn that from Isaiah 6:9-13 when the Lord says he closes ears, shuts eyes, and heardens hearts. And at the end of this verse he says when it will grow again-Isaiah says in chapter 11 who has the spirit to open the eyes and ears and hearts of the people. And in chapter 33:5-6).
So then by going back to 1 John 4:2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God. And every spirit that confesseth not Jesus Christ is come in the flesh is not of God: and this is that spirit of the antichrist, whereof ye have heard that it should come and even now already is it in the world.
We see why confessing Jesus is so important.
See the interesting thing that I noticed (and I dont know if this is important or not but it is interesting) is this: Acts 2:11 Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God.
(Then as you near the end of what happened you see this as Peter continued to speak the words of his Lord to the crowd)
Acts 2:37 Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost....
41 Then they that gladly received his word were baptized: and the same day they were added unto them about three thousand souls.
(What I had noticed here was all of it was about the word of God, Jesus Christ. Even the message given in tongues/other languages. And then hearts were touched afterwards, and people came to salvation of their souls).
(When the Lord does things like these, miracles, signs, wonders, it causes belief. In one place I had read before it said, prophecy was for the believers, and tongues was to cause belief in unbelievers *1 Cor. 14:22*)
Blessings