There's no checking. You just pour some in until it sounds like it's coming up the neck. It's a BIG reservoir so if you get it to the point of running out, it'll take an entire gallon bottle and more.
Either a physical problem with the button or a problem detecting the fob wherever you've got it when you're pressing the button. Try holding the ring in one hand and pressing the button with the other. (That is, not having the fob in a pocket.) Try replacing the battery. Now is about when I'd expect the SECOND battery in the fob to be dying or almost dead. (They last two years, and die shortly into the third year, for most people.) Physical problem is gonna be a repair job.