greek world

everyone who read the old glosses is amazed to see that in
every part of the world known by greeks/romans, the word are
all ending in "es, us, os, is, s" and so on.
It seems improbable that these have been the endings of the
words in all these ancient languages, that every language have
had these "es, os, is, s" , etc as endings for the word. It
seems more probable that the greek "greekysed" the words and
latins "latinised" the forms of the words.
I do not blame them for it, it seems to be something normal to
naturalize a word in your own speach.But in this case, I have
a question regarding this.
-how is possible anymore to know if there was an original "es,
os, is, s" or if there is a greek or a latin
naturalisation?How can we be sure that we have the right
pronounciation of the word and not the greecised/latinised
one?

many thanks in advance.

alex