As a man, how do you work out when it's ok to approach a woman? As a woman, how do you 'signal' to a man that it's ok for him to approach you?

At some point, upon some sort of eye contact I can get a hint if someone would entertain communicating with me....that's my signal.
If you smile and come across as friendly, you will be initially successful.
What and how you speak directs the situation from that point.

Eye contact and lots of smiles and conversation. I will ask him tons of questions 😀

It all just starts with a look, a smile, a compliment and a simple hello.