Tongues a sign
The question of tongues as evidence of the Holy Ghost, In times past I was taught this way, I put this out for concideration only. The question has been ask for scripture, this is what I used to teach.
Mat 12:33 Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.
Mat 12:34 O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh.
Mat 12:35 A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
Mat 12:36 But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
From the heart our mouth speaks, ever hit your thumb with a hammer what was your response? Most in the world would issue a swear word. Why? Its in their heart.
Back in Genesis when the people did not obey God and replenish the earth after the flood God came down and used tongues to scatter the people. Today God uses tongues to bring people together in his kingdom.
Jas 3:2 For in many things we offend all. If any man offend not in word, the same is a perfect man, and able also to bridle the whole body.
Note James here lets us know that if a person can keep his mouth in check and offend not someone he would be a perfect man. You and I both know this is near impossible without the spirit of God.
James goes on to tell us in the next several verses that our tongue is a evil member that cannot be tame. It is unruly and evil and full of poison.
So what happens when God fills someone with his spirit, or what happens when someone speaks in tongues. It shows that they have allowed God into their heart for that time so that they speak the wonderful works of God. (praises and worship.) This is not to say every time some one speaks in tongues it is God. I have found in my life many, myself included that memorized the word I said and would repeat them when I prayed. This is one of the reasons I stopped speaking in tongues unless I truely felt the move of God in my heart.
I hope this helps again this is the way I was taught and what I taught for years. Not neccesaraly what I teach today. But I put this out because it was scripture based.