I have never been able to figure that one out! As a result, I often have to pass up on entering any discussion, as I don't want to be rude by butting in, or by changing a subject not yet completed by existing parties to the conversation. I have been told that it's all in the body language, but how in the heck is there any body language when people are just standing or sitting there? No feet to point toward a door, nor any rattling of keys, or anything else to indicate an end in sight that I (as a high-functioning autistic) can recognize. It's a crap shoot when breaking into a group in conversation, and I just have to hope it isn't taken as TOO rude.
When I do start one, I have a terrible time ending it, as I have been accused many times of "bomb dropping" (posing a topic to discuss, and leaving before the other party/parties have a chance to fulfill a response)... I hate being that way, and I basically apologize in advance when leaving the conversation... Maybe I'll just say "Excuse me for any rudeness, but I just realized it's time for me to (whatever suits), and I need to go just now".... Maybe we can resume later?" That way, the other person can say "Just one more point............", and I'll listen and then know they were done, or they'll say "Oh, no problem... I didn't really have any further on that".
Charles