It is really important to ensure that you have a fully trained Electrician and not just a PAT Tester. If all of your appliances depend on an Earth for safety, and yet the Fixed Wire supply (i.e. the Socket) has a fault and is not got an Earth connection for some reason then your appliance will not be safe to use.
As for what Appliances need to be tested then this is not as complicated as it sounds. Any electrical appliance that has a plug and is intended to be connected to a wall socket or generator needs to be PAT tested. This includes items such as all I.T equipment, kettles, hair dryers, irons and desk lamps. It includes bigger items such as fridges, compressors, washing machines, photocopiers and vending machines.
All 110v volt equipment most commonly found on building and construction sites needs to be PAT tested also but on a much more regular basis. Items that run off of a battery such as laptops and drills do not need to be PAT tested but the battery chargers will need to be.
To be PAT tested, the electrical appliance will need to be switched off so that the plug can be opened and inspected (unless it is moulded) as well as tested on a PAT Testing machine. This is not always possible for some businesses that have sensitive equipment such as computer servers and CCTV equipment. In this case it is advisable to have these items PAT tested during a weekend or a planned maintenance shutdown period.