For a quick way to tell if there’s charge left in a 9 volt battery, briefly touch the contacts together with your tongue. If there’s juice in the battery you’ll get a tiny shock (and a nasty metallic taste). It’s not enough voltage to hurt you but it’s definitely not pleasant and you wouldn’t want to test a bunch of batteries this way.

I can’t remember exactly who showed me this technique but I’ve used it for years in checking gadgets like smoke detectors and guitar tuners. Sometimes the battery truly is dead but other times it’s just a matter of it having been jarred out of place and needing to reposition it so the contacts line up properly.