Weak gravitational lensing distorts the image of galaxies by making them elliptical and non elliptical, but this we see in every day effects. So how do you determine what is just the shape noise of the galaxies and what is the distorting of the galaxy/cluster due to dark matter?
Shape noise vs. weak lensing can be differentiated based on patterns in distortion among a large sample of galaxies. If distortions in multiple galaxies tend towards some central area, there's probably some weak lensing occuring. If it's random, then it's either shape noise, or the distribution of matter between the telescope and the galaxies is inline with the cosmological principle (homogeneous/isotropic), and the light from the galaxies are effected by multiple lensing objects.