My therapist helped me realize it after I spiraled into a 3-year major depressive episode that made me suicidal every time I tried to write. (not great when you write for a living.)
Everyone will experience it differently, and you can find some good things to look out for online. (WebMD actually has a brief rundown of it and a few things to look out for)
For me, it started out as extreme fatigue and feeling overstimulated constantly, even when I was doing nothing. Nothing helped me feel relaxed, though with hindsight, that's because I didn't actually give my brain a break.
What I did was jump from one task to another or from one overstimulating distraction to another, thinking I just needed to 'take my mind off things' when what I needed was to take some time off to actually rest and do things that brought me joy, not just things that kept me busy and distracted from my emotions.
Hence why my brain dumped napalm on what was left of my mental health.
After a while, all the hacks and coping strategies I had for my executive dysfunction stopped working, and I was struggling to do everything because I just couldn't figure out what to prioritize. It was like being numb but also on fire at the same time.
I've since clawed my way back to some semblance of mental health, but I'm now very aware of avoiding overwork and prioritizing rest and mental enrichment over keeping myself distracted. If I find myself being task-avoidant, I pause and ask myself why.
Is it simply a low-reward task that I find boring? Or is it causing me some form of distress that I'm not registering because I'm not yet at the point of a total meltdown? If so, why? And what can I do about it? Is it something I can ask for help with?
If you're worried about being burned out, chances are you're already feeling it, and your brain is waving a little flag that says, "Hey, we need a break."
And if it's not yet full-on burnout, your brain is still saying, "Hey, we need a break," and it's better to listen to it than ignore it. Which is easier said than done, I know, but it's vital skill to develop.
If you're up for some reading/listening, a book that really helped me was Burnout: The Secret to Unlocking the Stress Cycle by Emily and Amelia Nagoski.