well for the most part you can gauge dehydration by urine color, but b vitamins will color it yellow so coldheartbear is technically right. but like i said, for most instances it can give u a good idea. shit i can feel dry.
FYI: those drug screen passing drinks contain massive amounts of creatine to fuck up the test and massive b vitamins so your urine does not appear diluted after you drink a lot of water.