A.T. Robertson Commentary Luke 1:18

"And Zacharias said unto the angel, Whereby shall I know this? for I am an old man, and my wife well stricken in years." — Luke 1:18 (ASV)

Whereby (κατα τ). According to what. It was too good to be true and Zacharias demanded proof and gives the reason (for, γαρ) for his doubt. He had prayed for this blessing and was now sceptical like the disciples in the house of Mary about the return of Peter (Acts 12:14f.).