That was an odd sort of question. Zacharias's brows knitted together in puzzlement. "Shouldn't you know?" he asked him in turn. Because if Merlin didn't know when he was ready, how would anyone else know?
"You should know because.. you'll feel ready?" he suggested. "Generally, when people are ready to kiss - and do other things - they know they want to. Might be nervous, but not scared." Which was the big difference. You could be nervous about something and still want it.
"And with some things, you won't know until you try, really."