How can I tell if my child is getting dehydrated if he's been playing outside all day?

Color of face,thirst. If he has been outside all day and not been drinking he is behind on his fluids. Urine color will be dark yellow if he is slightly dry. He can be red faced and tired as well. It is helpful to educate him if he is 8 or over to drink often and younger to offer fluids regularly. He should be thirsty as well if he has not been drinking enough.