First of all, even if I was an atheist, I would have to agree with dad that a same-state past cannot be proven.
Second of all, we (dad & I) conclude that the past was different by simply comparing how the Bible describes it back then, to how it is now.
For example, the Bible speaks of no death prior to the Fall.
People die today.
By comparing a no-death-past to a natural-death-present, we conclude the past was different.
You can certainly disagree with that, but what you can't disagree with -- (and be correct) -- is that the Bible is silent on a different-state past.