I think I've been getting some warning signs lately. Basically I was out of town for work travel for 5 of the last 7 weeks and have been practicing with a vengeance after getting back home this past week since I have a competition coming up. There's a few signs I've grown to notice over the years:
forearm muscle soreness - I definitely have less stamina than before, when I play tired I make a lot more mistakes, and the soreness can last over a day. This is fine for now as I build up stamina again, and I can massage part of it out, but definitely something to manage leading up to the competition (will probably put a hard cap on daily practice time at some point).
slight pain in RH pinky tip from voicing - definitely a sign for me to take it easy and not play all-out so I don't bruise the bone. probably also a sign that I have the practice volume turned too low and I'm keybedding too much to compensate.
slight pain in RH back of hand - definitely a sign that I should stop for the day, cut back on practice, maybe take more time off. This might be related to the bicycle injury that served as the final push for me to quit piano in college but often pops up when I go through high-practice-volume periods (e.g. WIPAC last year)