I've been asking myself this for probably 18 months and I think I finally have something useful to say about it.
The framing I keep seeing online is: "burnout is exhaustion, hating your job is the job's fault." That's too clean. In my experience they blur.
Here's the test I wish someone had given me earlier. Ask yourself: if you got a new job tomorrow, identical work, different company, better manager, would you feel relief? Or would you still feel like you have nothing left?
If the answer is "I'd feel nothing," that's probably burnout. If the answer is "oh god yes get me out," that's more likely you're in the wrong place.
For me it was burnout, but I'd convinced myself it was the job. Blamed our incident culture, blamed the on-call rotation, blamed the manager. All real problems. But I left for a new role and felt... the same. Actually worse for a few months because I didn't have the old complaints as an outlet anymore.
Some actual signals I missed: I stopped caring whether things went well. Not stressed, just flat. I'd get home and have zero capacity for anything. Not tired, like a different thing. Processing gone. Things that used to genuinely interest me (new infra tooling, architecture decisions) felt like noise.
Differentiating it matters because the fix is different. If it's the job, switching works. If it's burnout, you might switch and bring the same empty tank with you.
I ended up taking 6 weeks off between jobs. Was lucky I could. Not the answer for everyone but it's the only thing that actually reset something for me.
Curious if others have a better heuristic. I keep seeing "just take a vacation" advice which, no, doesn't touch it.