  Massive high-redshift galaxies form in over-dense regions where the
probability of forming other galaxies is also strongly enhanced. Given an
observed flux of a galaxy, the inferred mass of its host halo tends to be
larger as its inferred redshift increases. As the mass and redshift of a galaxy
halo increase, the expected clustering of other galaxies around it gets
stronger. It is therefore possible to verify the high-redshift identity of a
galaxy (prior to an unambiguous spectral identification) from the clustering of
other galaxies around it. We illustrate this method for the massive galaxy
suggested by Mobasher et al. (2005) to be at redshift z~6.5. If this galaxy
were to exist at z~6.5, there should have been a mean of ~10 galaxies larger
than a hundredth of its mass and having z-band magnitudes less than ~25
detected as i-dropouts in the HUDF. We calculate an approximate probability
distribution for neighbor galaxies and determine that there is less than a
~0.3% chance of detecting no massive neighbor galaxies. The lack of other
massive z~6.5 galaxies in the HUDF image argues that the Mobasher et al. (2005)
galaxy is instead a low redshift interloper. We generalize our results to other
galaxy masses and redshifts.
