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Take the top off the toilet tank, then add 4-5 drops of food coloring to the water. Wait 15 minutes. The water in the bowl of your toilet should still be clear. If there's evidence of color, the flapper is leaking and should be replaced.


What the video doesn't tell you: resist the temptation to buy a generic flapper replacement valve from the big box home improvement store. Generally speaking, they are lower quality than what's already in your toilet, and you'll likely be replacing other stuff in there that's perfectly fine. All you need is a new flapper; a well-stocked plumbing store (the old-fashioned kind, with a counter) will have what you need. Take the brand and model number of your toilet to the store, or snap a quick digital photo of the inner workings. They'll probably even tell you how to put it in!

via expertvillage.com