The line between what is and is not creepy can be very thin. One of the plus sides of being a woman is that we are rarely accused of behaving creepy (even though we all do from time to time). However, for men... Well, they have to figure it out quickly. You can't just stare at a woman for an extended period of time, even if you find her hella attractive. It's not a compliment, sorry.
Unfortunately, there's no real guidebook for how to avoid being creepy, and it can depend on the relationship. All I can say is if you think there's any chance you're potentially making someone uncomfortable just stop <3.