Photometry

The density of stars near a MDO will rise very sharply in the region where the MDO dominates the mass. This basically comes from Jeans' equations expressing hydrostatic equilibrium, in the form

$${G\,M_{\rm DO}\over r^2}=-{\nabla \rho\sigma^2\over\rho}\,.$$ (11.1)

If the velocity dispersion does not rise very rapidly, $\sigma^2\approx$ constant, then the density must rise very rapidly, $d\ln(\rho)/dr\propto -1/r^2$ hence $\rho\propto \exp(1/r)$. So a rapid increase in density of light toward the centre could signify the presence of a SMBH.

However, the region where the MDO dominates may be very small, of order $r_{\rm MDO}\approx {GM_{\rm DO}/ \sigma^2}$. As viewed from Earth for a QSO at distance $D$, the angular extent will be $\approx 1{\hbox{\rm arcsec}} (M_{\rm DO}/2\times 10^8\hbox{$M_\odot$})\,(\sigma/200{\hbox{\rm km s$^{-1}$}})^{-2}\,(D/5{\hbox{\rm Mpc}})$. Such a small extent is hard to see from the ground (due to atmospheric seeing), and Hubble Space Telescope (HST) will be needed.

Now some galaxies do indeed show a dramatic increase in brightness - a cusp - toward the centre, even at HST resolution. Unfortunately, it is also known that the velocity dispersion $\sigma^2$ is not constant. So we cannot claim this proves the presence of a MDO - it's a good indication though.