pinkfox is right - by the time the rabbit "gives in" and shows signs of pain or distress, it's serious and often means impending death. They will hide signs until they simply can't hide them any more. Regular checks will show you more potential problems and milder problems than relying on the rabbit's behavior. One exception to that is their food and water consumption - if the rabbit lives alone you can often tell things from that, but obviously not in a communal living situation like young rabbits are often in.
The rabbits I've had that passed away didn't show symptoms until the last minute either. I had one that did not eat her pellets one night, was listless the next morning and I could feel a mass in her abdomen, in distress an hour or so later, and 3 hours later I put her down because nothing was helping - necropsy showed she had a complete intestinal blockage. I've had two die of enteritis, they ate normally but didn't want to move much and when picked up I could hear their bellies sloshing, they both died less than 12 hours after that without showing any other symptoms. I've had a couple complete mystery deaths, one a male that was just "off" for a full day before he died, and another a female that died with no symptoms whatsoever. That female could have been stress- and heat-related, but the male I would have chalked up to old age based on how he was acting if he hadn't only been 2 or 3.
Rabbits are strange and fragile creatures. Don't let unexpected deaths get you down too much, or worry you too much. They happen surprisingly often, even when you think you have done everything right. Do some regular checks so you know what "normal" is for yours, do some preventative maintenance when it seems right (you're doing that with greens and hay already), and try to learn something new every time you lose one. Hang in there!
