You don’t want to give a beautiful woman a physical compliment because it won’t have a huge effect on her.
Likewise you don’t want to tease someone who comes across like they might not be the most confident person.
The best way to do this is to suggest moving away from the dating site to a more personal method of communication.
Back in the day this was MSN Messenger, but nowadays you could use Facebook chat or Whats App.
Find something that you can grab hold of and use, whether that’s a specific piece of information or just a vibe you’re getting. It’s hard to figure someone out based on just a few words and a picture or two, but you have to learn to go with your instincts.