"The toilet will either start taking forever to refill or the valve is leaking, so there will be less water in the bowl," Siv says, "This can indicate it's time to replace the fill valve." ...

Lift the flapper valve so some water flows into the toilet bowl. If the water goes back down on its own, you won’t have to do anything else. Otherwise, you’ll have to get plunging. You’ll ...

It’s also possible to turn off water to the toilet, but if the shutoff valve hasn’t been used in a long while, it could be hard to turn and might wind up leaking when you turn the water back on.