  Most ultraluminous X-ray sources (ULXs) are argued to be powered by
supercritical accretion onto compact objects. One of the key questions
regarding these objects is whether or not the hard X-rays are geometrically
beamed toward the symmetric axis. We propose to test the scenario using disk
irradiation, to see how much the outer accretion disk sees the central hard
X-rays. We collect a sample of 11 bright ULXs with an identification of a
unique optical counterpart, and model their optical fluxes considering two
irradiating sources: soft X-rays from the photosphere of the optically thick
wind driven by supercritical accretion, and if needed in addition, hard X-rays
from the Comptonization component. Our results indicate that the soft X-ray
irradiation can account for the optical emission in the majority of ULXs, and
the fraction of hard X-rays reprocessed on the outer disk is constrained to be
no more than $\sim$$10^{-2}$ in general. Such an upper limit is well consistent
with the irradiation fraction expected in the case of no beaming. Therefore, no
stringent constraint on the beaming effect can be placed according to the
current data quality.
