Testing for bimodality is a rather tricky thing. A recent discussion of tests of bimodality on edstat-l, though, yielded a few promising leads relating to the Dip test.
This test was first describe in
J. A. Hartigan, P. M. Hartigan. The Dip Test of Unimodality. The Annals of Statistics. 1985;13(1):70-84.
P. M. Hartigan. Algorithm AS 217: Computation of the Dip Statistic to Test for Unimodality. Journal of the Royal Statistical Society. Series C (Applied Statistics). 1985;34(3):320-325. Article is behind a paywall.
There is software to run the dip test in R, Stata, and FORTRAN.