I hope I worded the question clearly. I was watching a documentary that delved into the Futhark runes and there didn't seem to be any debate as to what the multipurpose symbols were called and how they were said. I'd love to know how they know this!
In your case with the furthak it is quite simple, just speak with an icelander. Icelandic is almost the same as what "the Vikings" spoke around the turn of the first millennial.
The real scientific way is to analyse the words history and figure out what the origin or the root of the word is.
Let's put up an example that I have encountered in some researche and projects I have done.
During some researche on the execution process of people who refused to accept their sins or otherwise not "play by the book" in the 17-18th hundreds Denmark, we came across some execution speeches and songs. One of the things that was fun (for me at least) was yo try and read them out loud. As Danish shares a good amount of words with German (and even more back in the day) I knew that whenever I saw a word that was strange or I didn't know it was either an old German word or some strange old Danish word that is no longer in use. That gives an idea of which direction a word should go. Second up I know there where no unison rules or non at all for how to spell at the time. That gives an idea of how the person that wrote the text would have said things as he would have written it as he heard it. This night also give some insight in dialects.
The great thing about most of the western and west-asian world is that most languages stems from indo-european. That means that there always is some connection to another and hopefully more known language. That is a great help when trying to figure out how an ancient language have sounded. The older it gets the more connected they are. If it is a language of unknown origin it gets a lot harder though. Then you have to try and puzzle it together by studying if nearby languages have stolen words from it or if you can find influence from other languages in the unknown one. That is of cause if you can even read and make sense of what is written.
One way of analysing a language is to study the amount of vocal and consonant sounds used. That might also give an idea of where in the mouth the language is spoken.
I am by no means an expert on the subject as I only had a small course on this during my history studies and then again a small course of it during my one year of German language studies. But these are the general methods we (me and my study partner) used when we worked with projects where old sources in original language was present (mostly in various scandinavian texts).