This works great if you have access to a scale. I'm well versed in the question of circulatory shock. I've done clinical research on shock, participated in some of the sentinel studies on shock, and teach the subject to residents and medical students. With all due respect, I've probably seen more cases of medically significant dehydration and shock than most of the sensei here have seen white belts. Dehydration is not shock.
Shock is the global failure of cellular oxidative metabolism.
One form of shock is hypovolemic shock in which the circulating blood volume is low. Dehydration may cause shock but is hardly synonymous with shock. Other forms of shock may have normal blood volume but the volume is maldistributed, e.g. anaphylactic shock. In some cases of shock the blood volume may in fact be increased but the heart may be unable to circulate the blood and oxygen to the tissues, e.g. cardiogenic shock. They just aren't the same thing at all.