  We propose an observational test that can break the degeneracy of two main
classes of microlensing models to the Magellanic Clouds: (a) the lenses are
located in the Galactic halo, and (b) the lenses are located in the LMC disk.
The source stars in the latter (self-lensing) models tend to be at the far side
or behind the LMC disk, thus experience more reddening and extinction by dust
in the LMC disk than ordinary stars in a nearby line of sight. Clearly such
bias would not occur in the macho halo lensing models. We show that this
reddening effect is at a level readily observable for the present 30 or so
microlensing alerts fields, either with multi-band photometry from a good
seeing site, or more definitively with ultra-violet (UV) spectroscopy with
Space Telescope Imaging Spectrograph (STIS). Stars behind the LMC dust layer
should stand out as UV-faint objects (by more than 1 mag than average stars in
the LMC). HST can also resolve numerous faint neighbouring stars within a few
arcsecs of a lensed source, hence remove blending in these crowded regions and
build a reddening map to control the patchiness of dust.
