Depending on the severity of the leak, sometimes the leak is noticeable and sometimes not. A running toilet is an indication of a toilet leak, although, some toilets may incur a "silent" leak, which is harder to detect. Food coloring can be used to test toilet leaks. Just place a few drops of food coloring in the tank of the toilet. Allow a couple of hours to pass without flushing the toilet. If the food coloring has moved from the tank to the bowl of the toilet, then that is an indication the toilet has a leak.
A recommended way to check for a leak is to allow at least 4 to 5 hours where no water is being used, or no toilets flushed. Read the water meter before the duration and read it again after the duration. The read should be the same; if the read is higher at the end of the duration, then that is a possible indication there could be a leak present (if no water was used or toilets flushed during the duration.)