Next: After you perform the dye test if it appears that you do have a leak...
...Check the toilet flapper for a complete seal. Rubber flappers degrade over time. The flapper may be in need of replacement or a simple cleaning.
...If you notice that water is flowing into the overflow tube, you have an overflow leak. Adjust/lower the fill valve (or float) in order to lower the water line within the tank. The water line should always sit below the top entrance of the overflow tube.
...Make any adjustments/repairs