  We propose a new method to detect the gravitational lenses in the ongoing
microlensing experiments using medium and high resolution spectroscopy
(resolution > 6000). Since the radial velocity of the lens and lensed source
typically differs by ~100km/s, the spectral lines from the lens and source will
be shifted relative to each other by (1-2)A in the optical. We simulate
realistic composite spectra assuming different spectral types for the lens and
source and study the lens detectability as a function of the signal-to-noise
ratio, spectral resolution and lens-to-source light ratio. We show that it is
possible to measure the difference in radial velocity from an unequivocal
signature in the difference of cross- and auto-correlation functions calculated
from two spectra obtained at different magnifications. If the lens is brighter
than 10% of the unmagnified source we find that a spectral resolution of ~6000
and a signal-to-noise of 50 (at magnification maximum) are sufficient to
determine the relative radial velocity of the lens. At resolution ~40000, the
spectral resolution of high resolution spectrographs of 8-10m class telescopes,
the lens could even be detected at a brightness of ~3% of the source. Radial
velocities higher than 50km/s can be measured with an accuracy of a few km/s.
Practical difficulties and observation strategies are also discussed.
