This question exists, which is asking a similar question—but the accepted answer is not what I'm looking for (nor do I think it actually answers my question), indicating that the OP's intent in the linked-question and mine are very different.
That being said, I am unsure how to word my question differently (but open to suggestions), and am thus leaving this note.
As this answer explains, in many cases, a single character is not often used by itself. This question is asked w/ that knowledge in mind.
While learning vocabulary, I have access to a great many resources on meanings of words and characters. In websites like:
... words and single characters are treated identically. All characters have some [maybe historical] meaning, even though sometimes it isn't used, and even native speakers will say the character itself doesn't mean anything. In my short amount of time learning, there have been many instances where individual characters are used by themselves (对 , 是, 好, 说, just to name a few of the bunch).
How can I know whether or not a character can be used by itself? I understand that many times they cannot be, and the reason for that, but many times they can be—and I'm at a loss as to how to figure out when this is or isn't the case.