Many spatio-temporal systems can undergo instabilities, leading to the spontaneous formation of spatial structures (patterns). However, a range of cases exist for which the pattern itself is not directly visible because of technical or fundamental reasons. This is the case for the spontaneous formation of millimeter-scale patterns appearing inside relativistic electron bunches of accelerators. We demonstrate in this case how the study of responses to sine external perturbations can be used as a 'probe' to deduce the characteristic wavenumber of the pattern formation process. Experiments are performed in the UVSOR-II electron storage ring when the electron bunch is subjected to so-called microbunching instability, and the sine perturbations are provided by an external laser. The response is constituted of pulses of coherent synchrotron radiation, whose amplitude depends on the perturbation wavenumber. Experimental results on the dynamics are compared to numerical calculations obtained using a Vlasov–Fokker–Planck model.