By saying something like “you’re good looking/hot/cute”.
I kind of feel this is gender based. Not to stereotype but in my experience, many of my male friends would be fine with receiving a compliment from the opposite sex, Meanwhile my female friends think it’s creepy.
What do you think the difference exists?
The inappropriateness would be in the delivery of said compliment – how you say it, where you say it and, yes, who you’re saying it to. Six year old girl in her Easter dress at church (with family around, of course)? Compliment away! Leering at a cow-orker alone in the mail-room? You shut your mouth.
Everyone likes a well-meant compliment.
Also, what you are commenting on: “You’re looking cute today (implied: in that outfit).” is okay in a lot more settings than “You have a hot body!”