If her mouth does not get dry and her skin can be lightly pinched without making a temporary mark, she should be fine. The elasticity of skin test is a tried and true test of dehydration.

The other test is the color of urine. If it is bright yellow, she is fine. If it starts to get darker, is shows dehydration.

But, if she just drinks some water when she is thirsty, she should be fine. Thirst has worked since the beginning of man. It was not until the advent of bottled water that people started being so water conscious. Until then, we survived quite well just following our thirst.