Evolution is an optimization process, but it doesn't have "purpose" - it simply has byproducts that appear purposeful to humans.

How do you know it doesn't have purpose? Because you know how it works, and you know that nothing like "Make intelligent life." was contained in it's initial state in the way it could be contained in a Martian brain or an AI.

The dumb mating martian also did not leave the rocks with any (intuitively labeled) purpose.

I'm saying: Given a high knowledge of the actual process behind something, we can take a measure that can useful, and corresponds well to what we label intentionality.

In turn, if we have only the aftermath of a process as evidence, we may be able to identify features which correspond to a certain degree of intentionality, and that might help us infer specifics of the process.