And after a while came unto [him] they that stood by, and said to Peter, Surely thou also art [one] of them; for thy speech bewrayeth thee.
Note 4 at Mt 26:73: Some have thought that this statement about Peter's speech was a reference to Peter speaking in a holy manner that somehow connected him with Jesus and that, therefore, Peter began using profanity to distance himself from Jesus. However, as explained in note 5 at Mr 14:71, Peter's cursing was not profanity but rather an oath.
The speech that was being referred to was Peter's Galilean dialect. It was public knowledge that Jesus and His disciples were from Galilee, and that was what this man was referring to.