married, and living in a small rural town, I attended a local auction where I was able to purchase a full set of the World Book Encyclopedia for $4.00. This was the result of two conflicting diagrammatic understandings — one of use-value, the other of exchange-value — as to how this object should be valued. I found the one more appropriate from my own perspective and snatched up the "bargain". But how would we say which one was true or false? Or, assuming the diagrams themselves are not true or false, how would we judge which of the conflicting assertions was true or false?
With exchange-value the answer seems clear. The auction itself set the value at $4.00. More generally, someone might have paid that price knowing it was low relative to other markets. A collection of potential markets would then be the measure. But use-value is not so clear-cut. For one thing, it's not quantifiable, not without any number of dubious estimates, and for many that alone is enough to reject it as "unscientific". For another, even on a personal level, it still requires some fuzzy speculations as to just how useful a thing will be.
Despite their differences, however, both assessments reflect a reliance on purpose. Garages and backyards of auction junkies are full of objects bought at a "good" price but never subsequently exchanged (or used), and personally, I toted that encyclopedia set around for years without me, or my children, ever actually using it. Diagrams, first of all, seem to be assessed by whether they do, or do not, serve the purposes of those employing them.
My estimate of the use-value the encyclopedia set failed, and, thereby in a narrow sense, so did my diagram/concept of use-value. More exactly, though, the failure in this case only showed that something about my application of that concept was wrong. I still favor use-value to exchange-value in my capitalistic dealings, but through that application, and others like it, I have learned to be more skeptical about the future uses a thing might have.