How do we know finality is real?

Teleology or finality must be understood in terms of the metaphysical realities it is intended to indicate. As readers may know, Thomism agrees with Aristotle that being is diversified by potency & act (as a condition of defending the reality of many, limited and changing things against Parmenidean monism). Potency is nothing but a relation to some specifying act; a capacity for some currently-lacking actuality. This is the very essence of potency, to be intrinsically relative to act. The reference to act is potency itself.

The precise meaning of finality is nothing but this reference of potency to act, as Maritain notes here: the [intrinsic or essential, not accidental] "relation of the subject of change to the determination which actualizes it".

So, there is teleology in the universe insofar as things exhibit, as part of their ontological constitution, a measure of potency; and potency is nothing but a relation (the relation in which finality consists) to a determinate actuality or end.

This is clear in examples spanning the inorganic, vegetable, animal & rational orders. The variegated interactivity of the elements on the periodic table giving rise to physical phenomena; the photosynthetic capacities of green plants relative to the transformation of light into chemical energy; the unmistakable orientation of complex biological organs (eyes, lungs, gonads) to a determinate causal function (sight, respiration, reproduction): in all these contexts the fundamental relationality of potency to act, action to end is verified.

But rational & social life are also characterized by this intrinsic relativity of potency to act [re action to ultimate end]. Society is nothing but a multitude of individuals whose actions are collectively ordered to one such end. The act of intellection likewise has a specifying object or actuality: its own last end, the actuality which would complete its potencies (contemplation of God).

But we should temper our expectations concerning what we can know about the purpose of material things.

On the one hand, we know that insofar as something is in potency & can act, it will act for an end [since potency is relative to act, finite agents are in potency to operation].

On the other, the purpose or end of things aren't even close to always clear to us. We know things in the world from the "outside in" so to speak. The sources above say "via an inductive-deductive coming and going": we try to assemble a picture of essence by working backward from accidents (hopefully some of which are proper accidents which necessarily derive from and therefore “manifest” some essence or another). No doubt, as an a priori metaphysical consideration, every essence comprising potency relates to some end or specifying actuality. But how do we know what the exact essence of an ant is? We have to rely on experimental & observational knowledge via sensible accidents, which can never yield the essence or specific difference (nor its specific end) as such.

Thus, regarding the species of the physical world, we can't really assign with certainly the SPECIFIC end proper to each (though we can still know them & their ends in generic & transcedental notes: we may not know the essence of an ant, but we know as 'animal' it exhibits generative potencies for example).