The way someone stops smiling when you look away. How their answers become shorter, their laughter a little bit more forced for every time. You'll sometimes catch them with a distant look in their eyes, as if they were staring into the big nothingness that surrounds them. Completely gone. Lost. Alone. You might find them starting to distance themselves, making excuses, feeding you with little lies. "I'm just tired," or "I'm fine." They'll laugh it off, every single time.
You should pay attention to how they start to cover themselves. Hiding every inch of skin, so no one can see. They won't take your compliments, if so you'll get a simple "thank you" and they will take it all for lies.
Self hate is easily mistaken for self irony. Although, they will say it with a smile on their face, pay attention. Do not look away. Because they are nearly impossible to spot, unless you are one of them yourself.