  We investigate how the lensing potential can be measured tomographically with
future galaxy surveys using their number counts. Such a measurement is an
independent test of the standard $\Lambda$CDM framework and can be used to
discern modified theories of gravity. We perform a Fisher matrix forecast based
on galaxy angular-redshift power spectra, assuming specifications consistent
with future photometric Euclid-like surveys and spectroscopic SKA-like surveys.
For the Euclid-like survey we derive a fitting formula for the magnification
bias. Our analysis suggests that the cross correlation between different
redshift bins is very sensitive to the lensing potential such that the survey
can measure the amplitude of the lensing potential at the same level of
precision as other standard $\Lambda$CDM cosmological parameters.
