A gas leak can be dangerous and even if small can cost a lot of money over time. Where a leak is suspected a gasfitter will be able to use gasfitting tools such as a bubble leak detector or special leak detection fluid to test this. Signs that may indicate you have a leak include a faint smell of gas, or an abnormally high gas bill.
A test that you can carry out to check this is to turn off all the appliances in your house or apartment that use gas. Take a meter reading before you go to bed and then again in the morning when you wake up. If the reading has changed you may have a leak and should call a gasfitter to further look into this.