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Checking to see if something scrolls is way easier than looking at a design, calculating in your head if the margins look equidistant from one another thus deducing that it must be the bottom of the screen.I disagree, because you're not calculating anything. You just see the existence of a scrollbar and know immediately that the content exceeds the viewport and you can scroll. That's it. It's at least an order of magnitude faster than the alternative of "checking" because it happens instinctively without the slightest motor movement.
"Checking to see if something scrolls" means some form of finger or hand movement.
I know what you're saying though, because I do see people do it all the time. There is an awkward, to me, pattern of "I just started reading, so let's shake the content up and down to get oriented." It's just as foreign to me as people who highlight text as they're reading. Not my thing, but whatever. (On the highlighting behavior, I always figured it's both a visual cue and at least partially a matter of highlighted text becoming light-on-blue, which is easier to read than most web pages' black-on-white.)
> I always thought 'below the fold' was so overused or at least only for people who never use a computer, but I guess that's definitely wrong.
That advice was commonly head in web design and it wasn't really about people not knowing whether they can scroll or not. But rather, that visitors might just decide not to scroll before they leave your content because the first page is so uninteresting to them. It's because scrolling requires interaction that you're motivated to make the "above the fold" content grab their attention.
A behavior of "let's see if this scrolls by actually scrolling" is, in my opinion, an anti-pattern of bad UX.