Remember Duracell’s PowerCheck? The idea was that a strip built into the battery would show if the battery was good or not. Sure, you could always get a meter or a dedicated battery tester &#…
It failed often enough that it wasn't all that useful. A cheap battery tester is better. And for 9volts you can also use the tongue test, lol (don't really though). My grandfather used to do that all the time.
The tongue test works great. Be warned, though, that a full battery will make your tongue go numb. It'll feel like you have a big hole in the middle. Try it.
Ouch, I never had that problem, but I only barely touch it, lol. It's a little shock and slightly numb briefly. But fortunately I never had full numbness that lasted more than a second or two. But I haven't done it in a long time since I have a tester now. 😁
It's not much power, so it's not likely to cause major or permanent damage, but it may affect others differently and could cause burns if left on too long like if someone is less sensitive and doesn't think it's live.
And if the person is grounded and if they touch the hot side of the battery first there's a chance the charge could travel through the body rather than just the tongue. It's not enough to affect a heart, but might disrupt a pacemaker or other embedded device.
And of the battery is leaking, it could cause permanent damage from chemical burns from the alkaline and poisoning from heavy metals which while unlikely to be deadly with just one battery, heavy metal poisoning is cumulative across a lifetime.
So under ideal circumstances it is safe, but there are always risks with electricity and toxic chemicals, though relatively small.