"I suggest buying a digital multimeter. Decent ones can be found relatively inexpensively."
I just ordered one of these:
https://www.amazon.com/gp/product/B09W8LJPH5
It hasn't gotten here yet. I got it on the gushing recommendation of a friend who is kind of a meter freak, as in he used to work at Fluke and has a collection of high end Fluke meters, and he was all "How can they make a meter this good for $10". I'm just a dweeb who checks voltages, but wanted a cheap meter to leave at the cabin, and $10 is cheap :-).
(as far as hinky lithium ion batteries, I view them like hinky reloads. When in doubt, throw it out.)