How do I test electric cookers connected by twin & earth cable?

This would be tested as fixed equipment or an appliance, and should be carried out by a competent person. This would normally require disconnection of the appliance in order to complete the test safely. Competency in carrying out safe isolation in a more complex arrangement of work would be required. Safe systems of work should be observed at all times and that the tests completed are relevant to classes described in the Code of Practice.