Mistwell
Crusty Old Meatwad
I didn't see anywhere that the number of charges was supposed to be hidden information, so at worst, that's going to vary by table. But in general, yes, the players will go out of their way to avoid using the last charge whenever possible.
DMG (I think page 141). "The number of charges an item has remaining is revealed when an identify spell is cast on it, as well as when a creature attunes to it. Additionally, when an item regains charges, the creature attuned to it learns how many charges it regained."
So if you are attuned you do know. It would be one of the three magic items you can attune.