  We experimentally demonstrate phase retrieval of a single-soliton Kerr comb
using electric field cross-correlation implemented via dual-comb
interferometry. The phase profile of the Kerr comb is acquired through the
heterodyne beat between the Kerr comb and a reference electro-optical comb with
a pre-characterized phase profile. The soliton Kerr comb has a nearly flat
phase profile, and the pump line is observed to show a phase offset which
depends on the pumping parameters. The experimental results are in agreement
with numerical simulations. Our all-linear approach enables rapid measurements
(3.2 $\mu$s) with low input power (20 $\mu$W).
