Much like when I was in flying training and we were subjected to the altitude simulation pressure chamber so that we could each identify how we individually experienced the onset of hypoxia, I think it is important that everyone notates down the onset of burnout at home or at work.
For me, Stage 1 is when I start feeling myself getting irritable and snapping at those close to me for no reason at all. S2 is when I start having trouble sleeping, and get a really stiff neck leading to headaches. I've got to catch it at that point otherwise if it moves to S3 - crippling headaches, inability to focus on simple tasks or S4 - inability to even get out of a chair due to the feeling of extreme fatigue, then the recovery process is SO much longer and harder.
Keep a working journal, or better still, ask someone close to you that you can trust to note any out of character behavioural changes and advise you of them so that you can start to detect patterns and take action to mitigate.