Your landlord is legally required to have all gas appliances in the property checked annually by a Gas Safe registered engineer. This includes boilers, gas fires, gas cookers, and any other gas fittings. The engineer issues a Landlord Gas Safety Record (commonly called a gas safety certificate) confirming the appliances have been tested and are safe.
Your landlord must give you a copy of the certificate before you move in and within 28 days of each annual check thereafter. If they fail to do this, they're in breach of the Gas Safety (Installation and Use) Regulations 1998 and can face prosecution and penalties.
If you haven't received a gas safety certificate, ask your landlord for one in writing. If they can't provide it, contact the Health and Safety Executive (HSE) or your local council.