The scale is the dreaded measuring device most commonly used to check changing body composition; that is, are you getting fatter? It's not a good measure, though, since it doesn't really tell us anything about body composition--the ratios of fat, muscle, bone, and water that make up total body weight. Water can be particularly confounding since one cup weighs about half a pound, so drinking a beverage or using the bathroom can immediately change results in a measurable way. What we're really interested in is body fat percentage--that's the most useful measure of body "comp."
If the scale isn't up to snuff, what is?
Most body "comp" measures tend to be inaccurate, inconvenient, or both. Skinfold measurements can be reasonably accurate if administered correctly but results can vary with the calipers used and the person using them. They're also difficult to use on yourself, even if you know how to use them. Getting measured by someone else can be both inconvenient and uncomfortable: who wants a stranger measuring skinfolds on their nearly-naked body?
There are sophisticated tests such as DEXA scans and hydrostatic weighing, but these are tough to find (inconvenient) and not cheap although they are very accurate. Air-Displacement Plethysmography is another specialized option that can be more convenient and less expensive than the previous two while still being quite accurate. But unless you have some sacks of cash laying around, you're not likely to have a "Bod Pod" at home. (I tried one at the University at Buffalo quite a long time ago.) So still inconvenient.
Bioelectrical impedance devices try to measure body fat by sending small electrical signals and seeing how quickly they pass through the body. Since lean and fatty tissue offer different levels of electrical resistance (impedance), it's possible to estimate body-fat percentage this way, but not very accurately. Results can be skewed by hydration levels (just like the scale!) or recent meals. But it's cheap and relatively easy to implements, which is why these turn up on many bathroom scales and even exercise equipment.
Measuring tape used on various body parts--especially waist and hips--can give useful information about overall health (the waist-to-hip ratio) but not a good measure of body comp. It can also be tricky to self-measure. Pictures can work: a practiced eye can usually give a pretty decent estimate of body fat percentage. But looking at pictures of ourselves tends to be very subjective.
Ultimately, what most of us want to know is how our body composition changes over time. For that, measuring and pictures may suffice if you do them diligently and evaluate them objectively.
A handy-dandy alternative is to use the fit of your pants. It's a convenient and objective way to get practical feedback on body comp changes. This should come as a surprise to no one yet people still keep buying bigger pants. What if you didn't? What if you kept wearing pant that were too tight? At some point, they'd surely split open like they never do on The Hulk, but before then--and take this from someone who's worn the same size pants for 30+ years--they just get darned uncomfortable. This leaves three choices: wear different pants (we all have some that are more forgiving than others), buy bigger pants, or--and this is pretty radical--LOSE SOME FAT! Having to sit and walk around all day in uncomfortable pants can be a strong and constant reminder to eat a little better or do that next workout.
Note that this doesn't mean everyone needs to fit into "skinny jeans." Rather, this is a tool to maintain your current waistline, whatever it may be. Fitting into smaller pants as a goal is a different topic.
Speaking of jeans, they tend to be a good choice for Pants of Truth as they are not susceptible to much stretching. Obviously, pants with elastic waistbands are right out. (Sorry, Sensei Hutch...the Zubaz won't cut it.)
Beware the low-riders, including the unintentional ones. If you can't actually see the waistband while wearing the pants, you're doing it wrong. Keep the waistband a little higher; not Ed Grimley high but somewhere close to the navel usually works.
While a "belt of truth" can be used instead of pants, it's not as reliable an indicator. A belt can only measure waistline. I had some Pants of Truth that were tight in other places, so much so that I was sure they had shrunk. But I dropped some fat anyway...more than I expected. And voila! The pants magically "unshrunk." So stick with pants, especially if your extra fat tends to wind up on your hips and thighs rather than your waistline.
How about it...are you wearing your Pants of Truth? Do you dare?
Be seeing you.