I think my solution for carrying capacity is a common one. I like to tell my players, "Don't worry about carrying capacity. I won't enforce it unless it gets silly." However, I recently got curious and looked at my 7 Str gnome's character sheet. He's staggering around with over 200 lbs worth of gear. His maximum load is listed as 70 lbs.
If you don't enforce it a little, you get that sort of thing.
It's far worse than you think. Your gnome is a small sized creature, so your MAX LOAD is 52 lbs. But as a practical matter anything over about one half of your max load as a routine load is probably absurd. You certainly should be aware if you have anything but a light load.
If I found out a strength 7 gnome was carrying 200lbs of stuff around in my game, I'd have a fit. To me, this would be equivalent to a player failing to record damage on his character sheet when he was attacked, or arbitrarily doubling his hit points. Your gnome should be dead, and you have no real excuse because you ought to have known when you created the character that by having strength as a dump stat that you needed to pay special attention to how much you were carrying and consciously stayed aware of your weakness and inability to manage loads. If this was a strength 16 character and he discovered he'd hit 200lb of gear, I'd be like, "That was silly. Next time, pay some attention to your gear."
In the case of a strength 7 gnome with 200lb of gear, it would be beyond "silly" or "ridiculous" or we have a chuckle about how players always forget encumbrance, I'd be like, "Your character has had a heart attack from over exertion and is now dead and cannot be resurrected."
Whatever the litmus test is, you are so far beyond it that it's hardly reasonable to have this as a starting point regarding a good litmus test.
For me, it would be that you examine your gear regularly enough that you do not hit a heavy load very often and when you do, you try to do something about it when you do notice, and you at least pay some attention to the fact you are adding a heavy item to your inventory.
As with the comic you link to, I'm a little more forgiving with "The NPC is carrying the gear." because typically their ability to contribute to combat is already limited, but I would find it silly if the NPC had gone beyond max load.