You can get a Gas Safety certificate or Gas Safety Record by booking an appointment with A.R.C. and a Gas Safe qualified registered engineer will come to visit you and inspect your property.
They'll do a visual check on all your gas appliances, and pipework that are accessible, to make sure they're in good condition, and a test on the pipework to make sure there are no gas leaks
Letting a property that doesn't safely use gas is absolutely illegal and can be viewed as a criminal offence. The penalty for renting a property without a gas safety check includes a substantial fine and/or imprisonment, Invalid insurance. Up to £6000 in fines.