Yup, take your weight after exercise and divide by your weight before to estimate fluid loss e.g. 79/80kg = 1% loss of body weight. The weight lost is going to be water, and this will have happened through sweating and insensible losses such as moisture lost through breathing and heat.
To give you an idea, 1kg weight loss is equal to approx. 1L fluid lost. In order to hydrate properly consume 150% of fluid losses over the next 1-2 hours after exercise. Therefore consume 1.5L fluid within 1-2 hours after exercise e.g. x3 small bottles of water or isotonic drinks with some carbs and elec.