You may have a leak in your tank if you smell oil around the tank, in your basement, or close to your catch basins.
If your furnace seems to be using more fuel oil than you expect or can justify with the weather, your tank may be leaking.
Sometimes a tank will end up with oil and water in it, the oil will float on top of the water. To check for water, put some water-reactive paste on the end of a tape measure and feed it into the storage tank, the paste will change color if water is present. Any amount of water in your tank should be investigated, because oil will escape through the path the water came in.
During the summer, turn off your furnace, measure the level of oil in your fuel tank using a tape measure or some other measuring device. Measure it again, a few weeks later. If the level is lower, then the tank is probably leaking.
Residential heating oil is dyed red which makes it easier to identify. A leak may reach the bedding area of nearby underground pipes and find a path into groundwater or surface water.