Say a player asks "Do I feel like I've been poisoned?" Or "Do I feel like I have a disease?" Or "Do I feel like my leg is broken?"
I've considered a few options and have nothing that makes me think "Yes. That'll do."
Straight up Medicine doesn't seem exactly right: A character may be able to medically examine themselves for signs of a disease, break, etc. but if they're asking in the moment, having been hit with a piercing or slashing implement, or landed wrong after a big drop, they aren't making an examination so much a intuitive, self-awareness judgement call "can I put weight on my leg?" or "does something feel off with me since I ate?" type of decision.
Medicine-Charisma feels like a strong option if only because of a PHB description of charisma implying a level of self-awareness, but in context that seems more a reference to knowing ones own mind than ones body. Might work for a "Do I feel like my memory has been altered?" or similar but doesn't seem right for a quick personal assessment of physicality.
To clarify, I'm not asking what might give an advantage. Many things could (experience with medicine, poisons, etc.). And I'd likely allow such an attempt if applicable. So I'm not looking for a hard rule so much as a soft default...
Yes "NOTHING" is a valid answer. How badly am I hurt is often a trivial question to answer. And I'm not inclined to insist on rolling for everything, so won't argue that "Don't request a roll." isn't good advice here and in many cases, but there may be situations as listed in my question where one person with a greater self-awareness would be able to tell that they had been poisoned where another might not, or even to what degree of recognition AND there is likely to be an element of chance to that. Hence wanting to decide what the determining factor (or appropriate roll) might be.
I'm not suggesting I'd hide a sensation from a player. Example scenario: 2 characters enter a room. Each sits at a table and takes a sip from a different vial. They are told that one of the vials was poisoned. They aren't told the type of poison or its effects but let's say they're told one of them has only hours to live. The first thing each does is examine the vials. They give each a look and a sniff. No dice. Next thing they do is look at each other. One looks worried but neither seems certain. Next thing they do is consciously attempt to discern whether they feel at all unusual. This is something that, in my opinion, is not always a given. So this first guy; non too bright, low mental resilience, prone to fear, anxiety and not all that self aware (rolls 5-3 Medicine, Wisdom), so he ends up convinced that the itch on his knee is a symptom and panics. The other guy; cool as a cucumber, an expert herbalist and poisoner, and generally pretty in touch with his physicality (rolls Medicine, Wisdom with advantage - 19+4 and double proficiency), sits there and focuses on his respiration, notices that his heart rate is up, that the pinky on his left hand is a little numb and "oh s***. Yep. That's the old M5 playing up. Tasteless and odorless? Nothing local. Unless..." guy downs the rest of the vial. It induces vomiting. He stands covered in puke with a smile on his face. I'm happy this makes sense now. Medicine was obvious for a diagnosis. Just wondered if something else would have made more sense for the initial self-analysis.