Incremental Sheet Forming (ISF) has been the subject of research for over 15 years, but most experimental investigations have used relatively thin sheets (typically 1-1.5 mm). Do the benefits of incremental sheet forming continue to apply as sheet thickness increases? Furthermore, recent research has shown that there exists a wide range of other flexible forming processes that could be used for CNC sheet forming, as well as many older craft forming processes that could be automated. How can these processes be compared and evaluated? To answer these questions, a standard test is required. By analogy to control engineering, such a test is proposed here, in which a flexible sheet forming process is characterised by its spatial impulse response, defined as the change in sheet geometry induced by a brief application of the forming process. The spatial impulse can be represented as a change in shape or by a spectrogram of principal stretches. The impulse responses are determined for three processes; ISF; English wheel; power hammer. The effects of sheet thickness, prior deformation and impulse location are investigated. The results show that spatial impulse responses can allow a useful comparison between process designs, and indicate that ISF is sensitive to all three parameters, whereas the English wheel and power hammer are sensitive only to location.