Honestly, I have never relied on any bag check indicators for my bagged machines with the exception of the Aerus/Electrolux canisters. It is easier for me just to feel if the bag is full. I have found that often, even with dirty filter indicators on bagless machines, that the bag is full or the filter is dirty beyond a reasonable amount by the time the indicator says to take action. This is not to generalize all machines to having the same issue as some are more accurate than others, but a lot of them are not.
On a slightly different note, one of my favorite bag indicators was on some of the old Panasonic Jet-Flo models. Certain models featured a "full bag alarm" that made quite a unique sound when the air path was restricted enough.
