Drop the slide on an empty chamber and you get a "Slam!" but drop the slide on a loaded magazine and you get a "Kchunk!" (sorry, best description I could come up with). Or, you could drop the mag and check that it's one down, then top off. I don't have any pistols without either a loaded chamber indicator, or an extractor that sits proud when chambered, or a window where I can see the brass (with a flashlight if it's dark).