Poor skin turgor? Coated tongue? Urinary output down? No need for blood and/or urine osmols and you can forget about getting ADH levels, diagnosis of dehydration was based on well known physical signs. You know it when you see it.
Usually there was some issue compromising oral fluid intake. A quick, easy way to get a liter of fluid on board was hypodermoclysis which was seen as a bridge to adequate oral fluid intake. Very few patients had an IV line in place. The equipment included a 1000cc bottle of 0.9% saline and tubing which diverged into 2 tubes via a Y shaped bifurcation about 18 inches below the bottle. The two tubes were about 36 inches long and were connected to a 2 inch 20ga. needle.
The liter of fluid was hung from an IV pole or even attached to the traction frame of an ortho bed. The 2 inch needles were inserted very shallow and parallel to the skin of the anterior thigh with one needle per thigh. As a young whippersnapperrn, I asked an experienced nurse, "Why do we have to use such long needles for this?" The answer was "The fluid leaks out with short needles." Being the free spirit that I am, I once tried 1 1/2 inch needles and it worked just fine. Somehow, those 2 inch needles looked inhumane to me. Similar to those EKG needle electrodes that used to creep me out when sticking a patient with them.
After the needles were in place, the fluid was allowed to flow in without any clamps or regulation. This usually took about 3-4 hours. Old hospital rooms really did not have room for equipment like IV pumps or controllers which is a moot point as they were not yet invented.
One criticism I've heard of clysis is that it dumps liquid into an inappropriate fluid compartment (interstitial) and this renders it ineffective. When patients were badly dehydrated, they seemed to absorb subcutaneous fluids well. It did seem to work on a short term basis. Often a clysis would perk a patient up enough to begin taking oral fluids. Another advantage was since the needle insertion site was further from the patients hands, it was less likely to be pulled out. Since a hypodermoclysis site did not need observed closely like an IV site it could be covered with a dressing to prevent patient tampering.
Clysis was a rather crude way of replacing fluids that certainly had limitations, but, amazingly usually worked fairly well.