"Stares at them/glances" - do you not do this to people you don't like? I often find myself just people watching, so does that mean every girl I look at thinks I like her?
"Smiles in their presence" - smiling is a common way to greet and react to people
"Jokes around with them" - I joke around with everybody. And almost everybody tries to be funny with everybody.
"Stutters or acts nervous" - social anxiety can happen around anyone, or any girl, even if he doesn't like her
The other two are pretty good signs though. These too: -Physical contact -Eye contact + smiling without saying anything -Paying attention to her way more than he pays attention to others -Listening to her and seeming genuinely interested in what she has to say