I maintain a sense of humour at all times.

Humour goes first and comes back first. In a bad stretch it is the earliest thing to leave — jokes land somewhere off to the side, and you catch yourself producing a laugh a quarter-second late.

Then one day something genuinely stupid happens, a pigeon does something, and the laugh arrives before you have decided on it. That one is worth paying attention to. It is the most reliable early indicator anybody has, and it shows up well before any of the sensible measures do.

It does not have to be public, either. A private remark made in your own head in a waiting area counts entirely.