How is satiety measured?
Satiety is complex, and there are several kinds of satiety, all of which come into play in different circumstances. Things that decrease or halt our eating are called satiety signals. Different satiety signals are put out by different substances (carbohydrates, fats and proteins) as well as by things like gut stretching and other signals. There is also a psychological component to override satiety signals (company during meals, appearance of food, cost, etc.). The best measure of satiety may well be when a person stops eating a given meal. In lab conditions, the meals are carefully calibrated, but people (and animals) can feed as long as they desire to (ad lib).

Historically, satiety was thought to be the end product of serum glucose, available energy based on body heat, fat utilization by the liver, and the generation of ATP and other energy-rich molecules by the liver and/or brain. For the most part, these hypotheses have not withstood the test of time.

Three major signals influence food intake: satiety signals, adiposity signals, and central effectors. For the sake of simplicity, I'll deal only with satiety signals. Satiety signals (SS) arise from the GI tract and related organs during a meal, and influence eating behavior by activating peripheral nerves passing from the GI tract to the brain, or by recognition by brain receptors.[1]

As food interacts with the lining of the stomach and intestine, gut peptides and other signals that coordinate/optimize the digestive process are secreted. The signals that inform the CNS function as satiety signals. Different signals are secreted in response to carbohydrates, fats, and proteins, and it's the specific mix of signals that informs the brain as to what precisely has been eaten.[2] 

Cholecystokinin (CCK) is the most extensively studied gastrointestinal satiety hormone. It is secreted by cells in the duodenum and jejunum in response to ingested fat and protein in chyme (food plus gastric fluid). Giving CCK IV decreases ad lib meal size, while blocking CCK receptor sites will increase ad lib eating per meal.[3][4] 

Carbohydrates and fats as the most effective stimulators of Glucagon-Like Peptide (GLP-1) and Peptide YY (PYY). IV GLP-1 decreases meal size by promoting early satiety.[5] Within the CNS, PYY is detectable in the hypothalamus, medulla, pons, and spinal cord.[6]

Enterostatin is a protein which is broken down into a digestive lipase and a five-peptide fragment (Ala-Pro-Gly-Pro-Arg (APGPR) in humans). The appetite regulating effects of APGPR in the brain are specific to high-fat or fat diets but not towards diets rich in protein or carbohydrate.[6][7] 

Amylin decreases meal size in rats, probably through the same pathway as CCK. Oxyntomodulin (OXM) and pancreatic polypeptide (PP) are also meal terminators. Pre-prandial subcutaneous administration of OXM to overweight and obese humans over a 4-week period resulted in a significant reduction in body weight of 2.3 kg, compared with 0.5 kg for the placebo arm.[9]  

This is a growing field, and there is more suspected, and more to prove. But one can see that there is no one measure of satiety.

[1] That satiety signals come from the stomach and beyond was shown by "fake eating", that is, diverting swallowed food from the stomach by creating a fistula. If food is allowed to enter the stomach, satiety signals are elicited; if not, eating continues for a long time.
[2] Gastrointestinal Satiety Signals I. An overview of gastrointestinal signals that influence food intake
[3] Gastrointestinal hormones and satiety
[4] Rats with spontaneous mutations of the CCK-1 receptor (called
OLETF -Otsuka Long Evans Tokushima Fatty- rats) eventually become obese
during their lifespan.
[5] Glucagon-like peptide-1 promotes satiety and reduces food intake in patients with diabetes mellitus type 2
[6] Introduction to Gut-Brain Interactions
[7] Enterostatin inhibition of dietary fat intake is dependent on CCK-A receptors.
[8] Amylin decreases meal size in rats
[9] Gut-Brain Interrelationships and Control of Feeding Behavior