scythians sakas etc

i recently read mallory's book on the tarim basin mummies. he there
identifies the scythians sakas and saramatians as indoeuropeans. he
also says the modern ossetic people of the caucasaus are the modern
descendants of the scythians. however he doesn't say how this
identification is made except in reference to a few words. how do we
know what languages these people spoke and how are the ossetic people
linked to the scythians?

there are several languages spoken in the altaic region that he says
are descendants of the sakas speach. again how is this link
established.

another question i have is about the tocharians. it is occasionally
sugested that they were the kushans. so what writing or other
evidence do we have to establish this?