how do you know when you're allowed to touch/fondle/caress another adult person in a sexual/erotic/intimate/romantic way?

Are there defined 'signals and signs' that are supposed to be like cues to show you are allowed to approach and get intimate with someone? What do you understand about 'choosing signals' and how they relate to being allowed to get intimate with someone?

Does it vary between singles and couples? Does it vary between unmarried and married couples? Does it change when there are more than two people?