Try this simple toilet repair dye test; it’s easy to do, it’s fast and it’s inexpensive.
First, carefully remove the toilet tank cover, it is breakable and sometimes impossible to match if you happen to break it. Lay it on a towel or wrap it in newspapers or a small old carpet. Then, put several drops of food coloring into the water tank above the toilet (not in the toilet bowl :)).
Now wait 15 minutes WITHOUT flushing the toilet. After 15 minutes, if the toilet bowl is now colored, you can assume you have a leak. If there is a leak, more than likely you can fix it by replacing the flapper valve, an inexpensive toilet repair you can probably do yourself.