As far as I know no-one has ever directly measured the length of a relativistic object. However indirect measurements of Lorentz contraction have been made at the RHIC. This collides nuclei together at relativistic speeds and analyses the resulting shrapnel. The nuclei are far too small to see, but from the results of the collisions we can infer that the nuclei must have collided as disks not spheres:
So while the Lorentz contraction of the nuclei isn't seen directly it is inferred from the results.