I don't think it's obvious that you can't tell whether someone has free will from the outside. Considering the enormous success physics has had with explaining every other phenomenon from the outside, that seems like a very strong statement.

How would you distinguish decisions that were freely made from decisions made at random?

If we can't, then that says something doesn't it?

I just don't see how. It seems like a fairly obvious point that the two systems would be indistinguishable from the outside.

In that case, the pragmatists would say that the distinction isn't meaningful, that free will is a concept with no meaning, and end the debate there.

I don't think it's plausible to maintain that the question of whether or not we have free will is "meaningless". Any criterion of meaningfulness that derives that conclusion is probably faulty.