  Photon--photon scattering in vacuum due to the interaction with virtual
electron-positron pairs is a consequence of quantum electrodynamics. A way for
detecting this phenomenon has been devised based on interacting modes generated
in microwave waveguides or cavities [G. Brodin, M. Marklund and L. Stenflo,
Phys. Rev. Lett. \textbf{87} 171801 (2001)]. Here we materialize these ideas,
suggest a concrete cavity geometry, make quantitative estimates and propose
experimental details. It is found that detection of photon-photon scattering
can be within the reach of present day technology.
