When you have a meal, you get one 16 ounce glass of water. Your water intake is charted, and if you aren’t hydrating, you’re put on mandatory liquids (usually a combination of gatorade and water). While there are some exceptions to the rule, for the most part, you can tell by looking at people’s water who is anorexic and who is bulimic.
Bulimics, used to water-loading to help with purging, tend to drink all their water, and especially in the beginning, quickly and toward the first half of the meal.
Anorexics, for a variety of reasons (fear of stretching out the stomach, paranoia over calories, etc) tend to not drink their water.