In case you’re being genuine, a safe thing to do is to compliment someone on something they’ve done, not how they look (and especially not something they should do). So “that outfit really suits you” or “I love that necklace” is good, they presumably chose the outfit, “you’re so good looking/beautiful” could be creepy, they didn’t choose how they look and aren’t necessarily choosing to advertise it. Another good test is whether you would say it to a guy if you’re a straight guy i.e. you probably wouldn’t tell a guy he’s beautiful.