I like to think of myself as a hard worker, someone you can rely on to get a job done when it needs to be. Looking back, I think that's why it's hard for me to recognize the symptoms of burnout as they creep up on me. I've experienced different types of burnout as well that varied based on the type of stress that was causing it. Hopefully, this helps someone else out before they hit the burnout phase.
The first type of burnout is what I think of as physical burnout. This is when you know what needs to be done, you know how to do the task, but due to whatever circumstances, you get overworked to the point of burnout. In my experience, this can happen when deadlines are driven by external forces out of your control (or worse pop up unexpectedly), and/or you're not given the resources you need to complete the task by the deadline.
For example, at a previous job I was tasked with setting up a simulation lab with controllable light and sound effects. Initially, there was no firm deadline, but halfway through the setup, an artificial deadline popped up that required the room to be finished in days instead of weeks (with no additional help, of course). By putting in long hours, I was successful, and everything worked by the deadline. But by the end of it, I was exhausted and sick and had to miss a week of work after. The best part is the reason for the deadline never materialized. I had put in all that extra work and stress for nothing.
I've always been better working higher in the application stack - web and app development mostly. For one project, I had to go from a year of doing web development with React to low level C++ and an entirely new-to-me graphics framework. The project was five weeks long and it was one of the hardest, longest projects I've ever had the misfortune of working on in my career. There were multiple times during that project that I almost walked out and never came back. Fortunately, I had help on that project, so I had someone to struggle with, which helped a lot, and at the end of the day we were successful.
For lack of a better term, I call this mental burnout. Unlike physical burnout, this is where you may know what needs to get done, you don't know how to complete the task, and you're mentally drained from trying to make it all work. There's a mismatch between the skills that you have and the skills necessary to be successful. For me, this type of burnout is much worse. I don't mind putting in work if I know what I'm doing and how to be successful. When I've gone through in a situation like this though, it has been rough. There's a sense of hopelessness struggling through a task you don't feel equipped to do.
I wish I had some secret to share with you about how to recognize scenarios that are going to lead to burnout. To be completely honest, I'm still learning how to catch myself before I get caught in a burnout spiral myself. There are some things I've noticed that can be triggers in my experience - things like deadlines that appear or are moved up without the resources to meet the new deadlines. Being tasked with something you have no experience in (especially if combined with a tight deadline).
As for combatting burnout, one thing you'll need to learn is how you react to stresses and what your tolerances are since this is going to be different for everyone. Once you recognize you're potentially in a situation that could cause burnout, communication is key. I'm fortunate to be in a place where I have bi-weekly one-on-one meetings with my manager and can be honest with him, brutally so at times, when I see a burnout situation brewing. The bottom line is that if your manager doesn't know how you're feeling, then they won't be in a position to help - they're not mind readers. As I mentioned, everyone's threshold is going to be different. The other thing I think is important is be aware of how long you've gone on a project without a break - especially this year. I just got back from a week off because I realized I hadn't taken any time off in seven months. With everything else going on in the world, and working at home full time, I forgot to take time for myself and felt burnout creeping in. Now, instead of just going through the motions at work, I feel motivated to get stuff done.
In my career, I've experienced two types of burnout - physical and mental. The difference between the two is that in the former, you're overworked, and in the latter, your skills don't line up with the task and/or you're lost or confused. As an employee, you need to be clear with your manager if you feel any of these symptoms of burnout so that they can be addressed. If you're in management, it's your duty to your direct reports to push back against unreasonable deadlines and make sure that the talents of your employees line up with the tasks at hand. At the end of the day, this keeps everyone happy, healthy, and more productive than struggling through a difficult situation.
Finally: please, please take your PTO days! Work will always be there, so enjoy the time you have!