**How can you tell that you're attracted to that certain person?
And how can you tell that he is interested in you?**
It actually depends. If you two have a chat already like got to know your story like some friendly chat and you both are honest then you probably could. Their are even tons of people who have a 1 night stand. It's okay at the same time it's dangerous cause maybe that person has an HIV or some illness that will be pass to you