This sentence emphasizes that Zechariah and Elizabeth were far too old to have children by natural means. That would require God to work a miracle. (See: [[rc://en/ta/man/translate/figs-exclamations]])
Here, **know** means to learn by experience, suggesting Zechariah was asking for a sign as proof. Alternate translation: “What can you do to prove to me that this will happen?” (See: [[rc://en/ta/man/translate/figs-explicit]])