The scary thing about burnout is that you usually don't realize you are burning out before it's too late.

Personally, at least, I've worked on projects that just felt a little intense at the time, but after taking a step back due to holidays or hitting some milestone I realize I never want to have anything to do with it ever again. One project makes my stomach drop even today every time I see the code; Not because the code is bad, but how it takes me back to how miserable I was without admitting it to myself.

The biggest red flag I look for is when I'm tempted to work on stuff in my free time. When this starts seeming like a solution there's a serious problem with the project that needs to be addressed.