My first sign of dehydration is usually a headache.
Best tip I was given around hydration, is to pre-hydrate carefully. So while I would normally drink around a litre a day of water, on top of the usual fluid intake - tea/coffee/ food / wine/beer etc. If I have a middle on a Saturday, I would drink my usual amount Sun-Weds. On Thurs, would try and make sure I drank an extra litre of water, and on the Friday an extra two litres-ish on top of the 'norm'.
That way, come Saturday, I am already well-hydrated and not having to drink so much pre-game/half time and needing to pee for half the game!