And many respectable Christians believe tongues aren't event valid today.
In regard to tongues being a sign that you have the Holy Spirit . . . I went to a Charismatic Bible College. I was the only student in the college that didn't speak in tongues. I was walking with Jesus though. But I didn't have the "sign" that I had the Holy Spirit. Others who had "the sign" were sleeping around and one unmarried couple got pregnant. If that's "the sign" I figured that I didn't need it based on what I saw going on.
I believe that speaking in tongues is a real event and that it is a sign of being filled with the Holy Spirit. However, with that said, I also believe that some people fake speaking in tongues for various reasons and I believe there are other signs of being filled with the Holy Spirit.