Of course, a guitar is heavy, and a case without its guitar in it is a lot lighter, but I have a visual technique, that works pretty well.

I have a brightly colored, braided, woolen tassle tied to the headstock of my guitar. I also have one tied to the handle of the case. When I place the guitar in its case, I leave the tassle hanging out, where it can be seen.

I can tell which plain black case is mine, just by looking for the tassle on the handle. I can also tell, just by looking, whether the guitar is in its case or not, by the presence or absence of the tassle, hanging out near the head of the case.