  The redshift at which the universe was reionized is currently unknown. We
examine the optimal strategy for extracting this redshift, z_{reion}, from the
spectra of early sources. For a source located at a redshift z_s beyond but
close to reionization, 1<[(1+z_s)/(1+z_{reion})]<(32/27), the Gunn-Peterson
trough splits into disjoint Lyman-alpha, beta, and possibly higher Lyman series
troughs, with some transmitted flux in between these troughs. We show that
although the transmitted flux is suppressed considerably by the dense
Lyman-alpha forest after reionization, it is still detectable for sufficiently
bright sources and can be used to infer the reionization redshift. The Next
Generation Space Telescope will reach the spectroscopic sensitivity required
for the detection of such sources.
