What is a clue that an oil tank is buried outside a house?

The easiest way to tell if an oil tank in buried on the the property is the presence of a vent and/or filler tube coming out of the ground on the exterior. They are both about 2" wide.

Sometimes these are cut off so you can look in the basement near the furnace or boiler. You may see two 1/4" copper tubes coming through the wall or out of the floor. These are the oil supply lines. They can give you a clue where to look on the exterior.

This is a good article on the subject.