diphthongs

[Jacob Baltuch]

Greetings:

Apologies for repeating this query. I'm rewording
a bit just in case it works better this time.
(No luck last time)

There was some (entirely incidental) talk some
time ago from (at least two) people who knew
the phonetic realization of e/ai/o/au in
(or at some stage of) Vedic. Hence my question:
what is generally accepted as certain/probable
in that respect? (E.g. when -- i.e. in relative
chronology, relative to the texts that is -- did
the first component of ai/au become short? when
did e/o become phonetically pure vowels?)

I'm more interested in how you can know such
things [*] (the methodological aspect of the
question, to use a word which has recently made
a comeback) than in what the precise answers
are.

[*] in this specific case; I'm somewhat aware of
    how people have tried to go about arguing such
    things in other contexts.