Though St. Thomas never made it the subject of a formal treatise, the real distinction does find proof in his writings. The proof occurs in two principal forms. One starts from the objective (i.e. conceptual) distinction of essence and existence; the other turns on the consideration of received existence, the argument being that a received existence is really distinct from the essence receiving it.
a) The first proof, from De Ente et Essentia, is to this effect: Whatever is not included in the concept of the essence of a thing is extrinsic to the essence, hence superadded from without; but existence is not included in the concept of the essence of anything, therefore is added from without. The only exception, of course, would be a being whose essence is to exist - God. Because of its importance the proof wants the words of St. Thomas:
Whatever does not belong to the notion of essence or quiddity comes from without and enters into composition with the essence, for no essence is intelligible without its essential parts. Now, every essence or quiddity can be understood without anything being known of its existing. I can know what a man or a phoenix is and still be ignorant whether it exists in reality. Hence it is clear that the act of existing (esse) is other than essence or quiddity, unless, perhaps, there is a being whose quiddity is its very act of existing. And there can be only one such being, the First Being. . . . Consequently, this excepted, in every other thing the act of existing is other than its quiddity, nature, or form.5
b) The second proof, more frequently invoked by St. Thomas, contrasts created beings, whose essence and existence are found really distinct, with the first being, where they are identical - assuming the existence of this being, God. As developed by St. Thomas the proof, while admitting of variations, never varies in substance. One statement of it, from the Summa theologiae, is this:
Whatever a thing has besides its essence must be caused either by the constituent principles of that essence . . . or by some exterior agent. . . . Now it is impossible for a thing's being to be caused only by its essential constituent principles, for nothing can be the sufficient cause of its own being, if its being is caused. Therefore that thing, whose being differs from its essence, must have its being caused by another.6
Thus St. Thomas, who then goes on to draw the conclusion that in God, the uncaused being, essence and existence cannot but be identical; which is what he sets out to prove in the article from which this passage is cited. However, if the identity of essence and existence is an uncaused being, it necessarily follows that the caused being does not have this identity, its essence is not its existence; hence the distinction in view is established, if not explicitly, at least by implication.
To sum up, because the being whose essence and existence are identical is unique, all others necessarily bear the real distinction and are caused by the being which excludes it.