Yes, it's definitely impossible to tell...conceivably, I suppose, there could be some cable which is so badly made that when you tear it apart it's obvious. The twist rate could change from foot to foot, or the shield foil could be all messed up, or something like that which you could see. But the much more likely scenario is something like one of these (just a few examples; I could add many, many more):
(1) a wheel over which the wire is drawn just after passing through an annealing furnace is slightly out-of-round, and this is causing the wire, which is hot and quite malleable at this stage, to flatten out by a visually imperceptible amount at a regular interval, every few inches. The resulting irregularity causes a return loss spike at a certain high frequency and all of its harmonics.
(2) The extruder does not center the wire well within the dielectric. Consequently, the insulated wire is asymmetrical and, depending upon its orientation with relation to the other member of its pair, the impedance of the pair varies. This variability will have a periodicity to it as the pair twists, similarly causing a return loss spike at specific frequencies.
(3) When a pair was being twisted, slightly more tension was maintained by the wire prefeeder on the one member of the pair than the other. Consequently, one member of the pair is physically longer than the other, resulting in unacceptable intrapair skew (that is, the "plus" and "minus" sides of the differential signal fall out of sync with one another) and increased crosstalk.
These are just a few of the sorts of things that can go on. In fact, they happen in every cable; the question, which is hard to answer without the fancy test gear, is whether the process controls which have been put in place to minimize periodicity, center wires in dielectrics, maintain symmetry in pairs, and any number of other things, are keeping the overall tolerances in the cable down to an acceptable level. No wire is perfectly round; no dielectric is perfectly applied; no pair is perfectly symmetrical; it all comes down to controlling all of these variables at a microscopic level, while nonetheless being able to run the cable through the process at a practical speed. It's a daunting task, and that's why nobody builds HDMI cable stock in his basement--the whole task requires an enormous amount of practical engineering and in-process monitoring to yield a good result.
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