Nice to meet you, anyway. This is my very first comment in this group.
예쁜거 알지? As you know, this means 'you know how you are beautiful or attractive, aren't you?'.
So, I think he's intend is this you are beautiful or attractive. If you understood this sentence you can give your feedback or thanks comment on it.
BTW, since you want to know the response of Korean girl, let me share about it. In my experience as a Korean man, there is 2 types of the response. First is just say no I'm not, '아니에요..' like this. Second is saying 'Yes, I am. you know what' like this. Most of girls say '아니에요'.
Please consider this is only my personal opinion.
Have a nice day!