Most leaks discovered during the winter stem from toilet flappers. To test if your toilet is leaking, follow these steps:
1. You'll need a dye tablet or food coloring
2. Drop the dye tablet or a few drops of food coloring into the toilet's tank
3. Wait for about 10 minutes without flushing
4. Examine the toilet bowl for any colored water
5. If you notice color in the bowl, a leak is present