A test apparatus is configured to perform non-determinative testing of equipment. The test apparatus comprises a test computer arranged to automatically execute a non determinative test regime under the control of a test application. A network simulator connected to the test computer is provided with communication circuitry operable to communicate with the equipment under test. The network simulator is configurable into different network states according to the non-determinative test regime, and the test application is operable to control the network simulator to transition between a plurality of different network states. Data about unscripted communications between the network simulator and the equipment under test is monitored and can be analyzed to reach a test verdict.