We get this account from Herodotus and his historical volumes called "Histories"

The Battle of Thermopylae would have happened a good 40 years before Herodotus was born, by the time he was adult and writing histories possibly 80-90 years or so had passed, so we have to assume his sources are second and third hand sources, or written military records from that time possibly from first, second or third hand sources.

With that said, here is Herodotus' account of Ephilaites. Starting from Histories 7.213

"Then when the king was in a strait as to what he should do in the matter before him, Ephialtes the son of Eurydemos, a Malian, came to speech with him, supposing that he would win a very great reward from the king; and this man told him of the path which leads over the mountain to Thermopylae, and brought about the destruction of those Greeks who remained in that place. Afterwards from fear of the Spartans he fled to Thessaly, and when he had fled, a price was proclaimed for his life by the Deputies, when the Amphictyons met for their assembly at Pylai. Then some time afterwards having returned to Anticyra he was slain by Athenades a man of Trachis. Now this Athenades killed Ephialtes for another cause, which I shall set forth in the following part of the history, but he was honored for it none the less by the Spartans."

He continues on-

"Thus Ephialtes after these events was slain: there is however another tale told, that Onetes the son of Phanagoras, a man of Carystos, and Corydallos of Anticyra were those who showed the Persians the way round the mountain; but this I can by no means accept: for first we must judge by this fact, namely that the Deputies of the Greeks did not proclaim a price for the lives of Onetes and Corydallos, but for that of Ephialtes the Trachinian, having surely obtained the most exact information of the matter; and secondly we know that Ephialtes was an exile from his country to avoid this charge. True it is indeed that Onetes might know of this path, even though he were not a Malian, if he had had much intercourse with the country; but Ephialtes it was who led them round the mountain by the path, and him therefore I write down as the guilty man."

Here we see Herodotus writing down a story of a possible second culprit which has been repeated to him, but decides on Ephialtes as the guilty party based on what facts he knows of. Ephialtes is mentioned several times in the next couple of passages, but only matter of factly as the guilty party.

Was it 100% Ephilates? I guess will we never know without a doubt. Whether it was this specific person or someone else, the record we have still remains clear that someone tipped off Xerxes to a path around the mountainside. In these several passages it is mentioned the trail was often used during raiding and warring with nearby Phocians. So it seems plausible that the trail would have been known to locals to traverse the mountainus terrain.