‘ Then Boaz said to his servant who was set over the reapers, “Whose damsel is this?”
As he looked out over his land he saw Ruth following his reapers, and he turned to his overseer and asked who the woman was who was gleaning, and to what family she belonged. He possibly wanted to ensure that she had the right to glean (it was restricted to the needy), and even possibly in order to ensure that his overseer was doing his job properly. Part of his job would be to check up on the gleaners.
It is not surprising that Boaz did not know her, She would probably be well and discreetly covered up, and he may well never have seen her. Women did not move around as freely as they do today. So while he had made enquiries about Naomi's situation, it was unlikely that he had ever met Ruth.