  X-ray illumination of accretion disks is an invaluable diagnostic of the
structure of these disks because of the associated iron K$\alpha$ emission.
Here we point out that the resulting reflected spectra depend very sensitively
on the geometry of the X-ray source, and that this fact can be efficiently used
to test these models observationally. In particular, we discuss three different
accretion disk geometries: the ``lamppost model'', accretion disks with
magnetic flares, and the model with a full corona overlying a cold thin disk.
We show that in the case of the lamppost model, unless the X-ray luminosity of
the central source is larger than that of the cold disk by a factor of 10 or
more, a significant fraction of iron in the ionized skin of the disk is in the
hydrogen and helium-like ions. Because these ions have large fluorescence
yields, the resulting reflected spectra look strongly ionized, with Equivalent
Width (EW) of the line {\em increasing} with X-ray luminosity $L_x$ up to the
maximum of $\sim 500$ eV. This situation contrasts to the magnetic flare model,
where the large X-ray flux near flares completely ionizes the skin of the disk
and thus the resulting spectra appear to be that from a neutral material. The
line EW in this model {\em anti-correlates} with X-ray luminosity, and becomes
arbitrarily small when $L_x$ is a good fraction of the Eddington luminosity.
Finally, in the full corona case, due to the additional pressure and weight of
the corona, the gas pressure (and its density) below the corona is always large
enough to make the gas very cool and effectively neutral. No highly ionized
skin forms in such a model. If the corona is Thomson thin, then EW of the line
does not depend on the accretion disk or corona luminosities for the full
corona model.
