The easiest and most reliable way of checking your oil level is to dip a clean stick in to your tank. To avoid contamination make sure you use one that is used solely for your oil tank. Take the cap off the tank, and put the dipstick right down to the bottom of the tank. When you take the stick out, the oil will leave a mark, indicating the level of your oil.
The least effective and most unreliable ways to check your heating oil level are shining a torch in or tapping the tank. Your may have a watchman, a digital monitor that plugs into an electric socket inside your house - these are not always entirely accurate either but we'd recommend you order when the digit reaches 1 or 2 bars. Most systems are set to flash at this level which is a visible reminder to reorder. But don't panic or turn off your heating as you still have oil left in your tank.
To be sure though stick with a stick! Just make sure it’s clean and don’t drop it in.
Whatever method you use, we would always recommend you don't let your level get too low so as to prevent a delivery stirring up any sediment that may have settled at the bottom of your tank.