First, especially if you are not the orginal owner of the house, check with the township to see if there's anything on file about removal and/or filling it in (called decommisioning). If nothing comes up, and there was previously oil heat in the house, sometimes it's easy to find out if the oil tank was in the basement.
However, the only way to find out if there is/was an oil tank in the ground is to pay for a tank sweep from a reputable tank remediation company. If you find a tank, and there was no record of proper decommisioning of the tank. then pay for a soil evaluation to see if it leaked.
Despite what Jaques said, there are very strict DEP rules and procedures on removing and/or decommisioning an inground tank, and some municipalities require the total removal of the tank.
Here are some red flags which may help you figure it out, check and see If you have any pipes (filler and/or vent) around the perimeter of the home or look for disconnected thin lines in the crawl space or basement that may have been cut, those are usually indicators that their was oil previously on the property.
The one way to find out for sure is to have a tank sweep done there is a local firm called Hydroscience that can do this for you. They actually took care of my uncle's property on Central Blvd in Bayville he had an underground tank, they will know exactly what to do. There website is http://www.Hydroscienceinc.com and the phone number is 732-349-9692. If you should have any further questions or thoughts just let me know I would be glad to help out.
The Ocean's Six Group
REMAX Real Estate LTD