The first step in determining which optical microscope technique was used is to look at the general gray-value of the background.
White
Gray
Black
If it has a white background, and often multiple colors, it is a
BRIGHTFIELD image.
If it has a gray background then it can be either Phase Contrast, DIC, PlasDIC, or Hoffman Modulation Contrast.
If it has a black background than it is either Darkfield, Polarized Light, or Fluorescence microscopy.

(on to analyzing these images)

Look for the Phase Halo
High magnification
Phase Contrast
Phase Halo Present
Phase Halo not present

3D appearance

High magnification
DIC
Medium magnification, works with plastic
HMC
Medium magnification. Works with plastic
PlasDIC