You can say:

"My body's telling me I'm stressed."

"My body's telling me this isn't sustainable."

"My body's telling me no."

Internal experience is very important for things like this. Citing it is a valid response, and your friends ought to respect that, even in an "ask culture" where they are welcome to push you and ask hard questions.

Personal effort does generate stress (sometimes distress, sometimes eustress). Although a doctor can use crude tools like e.g. checking your blood pressure, you have a much more sensitive ability to understand your body and stress level than anyone else. Most of us have less of that ability than would be ideal, so we should value and cultivate what we do have.

That said, "this is stressful/unsustainable" doesn't mean that it was a bad idea to give that thing a try. Like you said, "have to try it to know it." Nor does it necessarily mean that it would continue to be equally stressful/unsustainable if you were to continue. New routines come with a transaction cost of added stress. Sometimes if you can get over that hump, it gets better. Sometimes not.

Even "my body is telling me no" about something that would not in fact get easier with practice doesn't necessarily mean that your productivity is capped at that level. It just means your "working harder" is capped at that level, currently. "Working smarter" is sometimes an option. And your personal budget for stress will wax and wane depending on outside factors. E.g. sleep, nutrition, security, social affirmation, etc. We are still in a pandemic, and many coping methods for replenishing ourselves have been unavailable for a long time. So maybe now just isn't the time.

I think it was good that you listened to your body, and good that you're asking these questions about the value and nature of personal experience and how to communicate it.

To answer the question of "how to describe internal experience": you could practice describing felt senses in more detail. For example, recently when I found my mind resisting the idea of doing something, I said "when I think of doing this, it feels like there's a part of my mind that says NO, and then I have a sense of there being a brick wall in front of me and it feels like if I try to push through, I'll just end up with a splitting headache". This was literally my experience.

To answer the question of "how to reliably signal internal experience": I'd say you can't. If you are looking for something that will always convince your friends of your experience, then there is no such thing: they could always believe that you were faking, or maybe not even faking but somehow subconsciously deluding yourself. Which you could be!

To believe your report, your friends have to have at least some genuine curiosity for, and openness to, your experience. If your friends don't have that, then - as others have mentioned - it would be better to look for better friends.

To answer the question of "what to do when I think I am doing my best but an outside view suggests that I am being needlessly defeatist": I think that in this case, even if the outside view was right, the best answer would not necessarily be to force yourself forward and work harder. 

Well, it depends on the circumstances - maybe you have something left undone that really needs to be done now for you to pay your rent next month, in which case, yeah probably just push yourself.

But in general, this kind of situation means that a part of your mind has information that makes it believe it is an important priority to stop you from doing whatever it is that you feel defeatist about. If you force yourself through, that may work in the short term, but the mind will react to that by noticing that you are doing something that it perceives to be dangerous, and increase the amount of resistance until you become unable to continue pushing through the thing. (If the resistance is mild, this might not be true, especially if pushing through gets you something that feels genuinely rewarding to counterbalance it; but often it is.)

In that case, what you want to do is not to push through, but take the time to find the source of that resistance and investigate why it is that your mind considers this to be a bad idea. If it's mistaken, it can be possible to reconsolidate the emotional learning that's blocking you. Though I suspect that in a lot of cases that lead to burnout, it's actually the other way around: you are doing something because a part of your mind has the mistaken belief that doing this will lead you to something that it is optimizing for, with the rest of the mind throwing up resistance because it knows that fact to be mistaken.

I think "friends" is too broad a category to explore options for this.  I also suspect it's not the right question to ask.

For close, long-term friends, the question shouldn't be about signaling, but about communication with them, and with yourself.  "I am at my limit" can be a condition that you should seek professional help to figure out if that's a permanent limit, or if it can be exercised and increased.  Depending on the friends' group culture, they may or may not be able to assist in that recommendation, and are simply expressing it as "that's way less than most people can do, you're hurting yourself by accepting it before you've explored every option to increase it".

For more distant relationships, like work-friends and friend-acquaintances, signaling gets much more complicated.  Saying "I'm at my limit" is also saying 'I'm not a very good ally - my limit is low and you shouldn't invest much in our relationship".  Many people look for alternate explanations for their lack of spoons in these contexts - too busy with other things, wiped out from other obligations, etc.   These are true but misleading, but make it a lot easier to maintain the acquaintanceship without them trying to fix you without knowing you.

The specific signals and communication of your emotional and physical state and it's impact on your interactions with people can vary EXTREMELY widely.  In some groups, "I'm out of spoons, I'm going to take a bath and watch Netflix rather than our planned game night" is understood and accepted.  In other groups, it would be an affront. Many cultures allow "migraine" as a somewhat-frequent reason for canceling, many have varying limits on when you'd be expected to "suck it up".