How did they know that Moses who stepped forth to assist them, was an Egyptian?
Maybe he had an Egyptian accent, maybe he was wearing Egyptian-style clothing. Maybe he told them where he came from.
It hardly surprises us that two verses later the stranger from nowhere falls in love with one of the daughters and marries her. Highly romantic, but it doesn’t really explain how they knew he was an Egyptian.
Perhaps he had such manners that the girls knew at once that he wasn’t a local. More importantly, he was so helpful that they thought, “No-one from around here does so much for other people”.
Why do they call him an Egyptian? Possibly their father, with his own Egyptian experience, had told them that Egyptian young men were polite and well brought up.