You should be able to find out if your tank is running low by examining the level indicator tube. It can be found on either the side of the tank or on the fuel volume gauge. It should tell you how much oil is available. Don’t let it run too low as you don’t want to be stuck on a cold winter’s night with no heating.
If you can’t see the indicator tube it may have become worn and clouded as this often happens after it’s been used for a few years. If so, it might need to be replaced. As a rule of thumb, you should probably check your heating oil supply once a month or so. Mine tends to deplete every 6 weeks or so, at which point I contact Solo Petroleum, my supplier of heating oil in Yorkshire. On particularly cold months, it can deplete much quicker.