  We propose that relative variability on short time-scales of the multiple
images of a lensed quasar, after removal of the time delay, may be caused by
hot spots or other moving structures in the accretion disk crossing microlens
caustics caused by stellar mass objects in the lensing galaxy. Such variability
has been reported in the two images of 0957+561. The short durations would be
due to the high rotation speed of the disk (v/c ~ 0.1), rather than planetary
mass objects in the slowly moving (v/c ~ 0.001) lens. This interpretation could
be confirmed by finding periodicity, or correlations of the spectral and flux
variations due to the Doppler effect in the disk. We also propose another
signature of stationary accretion disks (with no intrinsic variability): the
gradient of the magnification over the accretion disk should cause a relative
color change between the images whose sign and amplitude are correlated with
the time derivative of the flux difference between the images. Other color
terms induced by the radial variation of disk colors are of second order in the
magnification gradient. The methods proposed here can be used first to verify
that accretion disks near supermassive black holes are the source of the
continuum radiation from quasars, and then to study them.
