I do Landlords Gas Safety Certs all the time and the fist thing to do is visualy inspect the installation inc. all pipe work to make sure it is correct, Safe and meets regulations. Then you carry out a gas tightness test. This involves attaching equipment to the gas meter and checking for 'let by' at the main controll valve & leaks, working pressures and standing pressures in the whole installation, we also check that the bonding is attached ( Green and Yellow Earth wire ) to the pipe leading from the gas meter within 300mm from the meter. Then we move on to the appliances each one should be visualy inspected and also with the right equipment attached checked for correct working pressures, Leaks and saftey. We also check for any safety devices that are fitted (FFD,s & FSD's) to the appliances are all working corectly. Boilers and fires have to be checked for the safe evacuation of the products of combustion aswell the afore mentioned proceedures. Once satisfied that everything is in order we have to fill out a certificate in triplicate. One for the Tennant to keep, One for the Landlord and one for our own records incase of any come back. It will be dated and signed by the Corgi Gasman and either the tennant or the landlord ( whoever is there at the time of the inspection). This is the standard tests we carry out and by law in rented accomodation and has to be done every twelve months.
I hope this sheds some light on you're query and helps you out. Any more questions then feel free to ask :o)