  We propose a scheme to detect analog Hawking radiation (HR) in an atomic
Bose-Einstein condensate (BEC) through measuring the diffusion of a dark
soliton. The HR is generated by changing the transverse trapping potential of
the BEC to obtain a background flow, which is subsonic in downstream and
supersonic in upstream, satisfying the condition of black hole horizon. When
the system is in thermal equilibrium at Hawking temperature, a dark soliton is
created in the upstream. Due to the influence of the HR, the motion of the dark
soliton is similar to a Brownian particle and hence exhibits an apparent
diffusion, which can be measured and be taken as a signal of the HR. Since the
dark soliton is much "heavier" than Hawking quanta, its diffusion is much
easier detectable than the Hawking quanta themselves.
