Do you believe speaking in tongues is a certain sign of salvation?
Yes. Peter believed that. That's how he knew the gentiles had received the gift of the Holy Spirit
just as they did. Being able to speak in tongues shows that you have received the gift of the Holy Spirit in salvation.
On this side of the cross, only the body of Christ has the Holy Spirit in the empowering of the gifts of the Spirit. Yes, the gifts can be faked, but if hang around tongue talkers long enough you eventually can tell the real gift from those who are faking it. Tongues just happens to be the one gift that we can use to determine, generally speaking, who
really has the Holy Spirit in a spiritual gift. You can't definitively determine who has the Holy Spirit by the other gifts with the exception of workers of miracles--a truly rare gift in the western church (credit cards, insurance, etc. pretty much make it so we don't 'need' that gift).
What about the person who never speaks in tongues, is that a certain sign he is not saved?
Thankfully, no, because I do not have that gift. I know a person close to me who had that gift (trust me, it was genuine) and they fell away into unbelief because of the trials and tribulations of growing up into the faith (soil #2 in the Parable of the Sower). They will have been away from the Lord in unbelief for 16 years next month.
In my journey for spiritual significance in the Full Gospel church I learned that the
fruit of the Spirit is actually the sign of salvation that one should seek for confirmation of having the Son of Promise growing inside of them (like Sarah did after she received the promises). We look in the mirror and we see Jesus growing more and more inside of us via the traits and characteristics of the fruit of the Spirit and so we know that we have Christ inside of us in salvation. A growing of Christ in us that will continue,
if we keep believing, until Christ in us comes to full term at the Resurrection.
If you love the Lord, and because you do, you seek to be justified by your works (as James uses the word 'justified', not Paul), seek to be characterized by patience, self control, forgiveness, calmness, peace, and so on. Those traits of the Spirit justify you (show you) to have the circumcision of the heart by the Spirit of God in salvation. The more you grow up into those things the greater your assurance is that you have believed and are continuing to believe in a genuine salvation. That is how we are to seek to make our calling and election a sure thing:
"10Therefore, brethren, be all the more diligent to make certain about His calling and choosing you..." (2 Peter 1:10 NASB)
"11And we desire that each one of you show the same diligence so as to realize the full assurance of hope until the end, 12so that you will not be sluggish, but imitators of those who through faith and patience inherit the promises." (Hebrews 6:11-12 NASB)
All this is not a works gospel.
None of this is about earning your salvation, but it surely shows how works must be an integral part of a genuine salvation experience--like getting wet must be an integral part of going swimming. If we claim to be swimming in Christ's baptism of the Holy Spirit we should be able to demonstrate the wetness of that baptism in how we behave. If we can't, we need to seek to do that so we can know for sure if we are genuinely saved or not. As James says, the claim of faith that doesn't have works attached is a faith that can not save you at the Judgment. The absence of the fruit of the Spirit showing in your life being the evidence that you do not have the Holy Spirit in a genuine saving faith.