Isn't it obvious?
Then you probably are.
I should just accept the complement and not worry about context, but I'm not overly confident and always wonder if there's a hidden tease.
In person or online?
If you think they are cute or hot you could give them a big hug and maybe a kiss on the cheek. If you don't then just say thanks.
I thank them for the compliment, even if I know they mean my flesh is hot to the touch and I'm probably racking up a fever...