More energy and less need for sleep are the best indicators, probably, as most/all mentioned. Being quicker, feeling/being more vigilant. More challenges, lessening your chances of success, feeling exhilarated by it, including opposition (though you might (first) get very irritable and agitated). Sometimes feeling you can't juggle/control it all, but this fear disappears (or you get depressed, maybe just shortly before moving on).

Irritability itself can also mean depression. Difficulty with keeping a routine is also a sign for both. Being late or almost late for work/school or some appointment can also be indicative of both.

Feeling unfree. Sleeping longer or being more tired than usual. Losing sense of what is meaningful and what isn't. Being slower at doing things. Just like depression, but subtle.

Edit:
Anxiety (even mildly) fluctuating might be the best sign that you are bound to face anxiety sooner or later that is too much, leading to a long depression. But don't try too hard to slow down or you will get more anxiety fluctuations. Try to avoid stimulation/opposition instead. Gradually do less, but don't get understimulated or it will also cause anxiety, a feeling of losing control. Always be vigilant to keep a balance. Look how others look at you, whether they notice a change, look (mildly) surprised.