  In this paper we will show that photon-photon collision experiments using
extreme lasers can provide measurable effects giving fundamental information
about the essence of QED, its Lagrangian. A possible scenario with two
counterpropagating ultraintense lasers for an experiment to detect scattering
between optical photons is analyzed. We discuss the importance of the pulse
widths and waists, the best scenario for overlapping the beams and signal
detection, as well as ways to distinguish the signal from the noise. This would
need a high-precision measurement, with control of temporal jitter and noise.
We conclude that such experiment is barely feasible at 10^{23} W/cm^2 and very
promising at 10^{24} W/cm^2
