I just came across a dehydration chart at one of the local sports hall.
Dehydration is one of the danger of a high intensity sports especially in a humid and warm conditions. This is evident in an enclosed indoor hall, where badminton is played.
An advisory from Mayo clinic here talks about the symptoms and dangers of dehydration.
A simple way is to monitor at your urine colour...
There also many sports drinks, for example brands like Gatorade, 100 plus etc. They serve a particular purpose of replenishing lost fluids and provide some carbohydrates for fueling the body.
Above: some of the sports drink available in the local market: Pocari Sweat, Sportswater and H-Two-O.
I actually find sports drink contains more sodium that is lost as is refered to in this article.