Both the oil you buy and the tank you store it in are a big cost to any household, if you have a problem you could end up seriously out of pocket.
OFTEC, the body which represents suppliers, engineers and installers, suggests the first thing to do is for an Annual Tank Inspection. This should be carried out by a professional technician, a list of local technicians can be found of the OFTEC website.
There are a few simple visual checks you can do yourself: check for corrosion, cracks and discoloration and make sure no debris can get into the tank to affect the oil such as trees, leaves etc. For further information this useful booklet gives you guidance on oil tanks, storage and regulations