The first step when a phytolith slide is put under the microscope is to recognise which ones of the observed particles are phytoliths.
Phytoliths are made of opal silica, which is amorphous and has a variable water content (SiO2·nH2O). Therefore anything under the microscope with the refractive index (RI) of opal silica (around 1.42) can be in principle a phytolith. If phytoliths are mounted in a higher RI they will have a distinctive greater relief than the mounting media. However, the observation of a series of characteristics can further help the researcher in correctly identifying a phytolith from any other amorphous silica particle (at least most of the time).
This set of characteristics should help in correctly identifying a phytolith. However, sometime volcanic ash particles can be confused for phytoliths. These particles are a common component in soils and sediments in areas where there are or there have been active volcanoes.
See the following illustrations to train your eye on real and fake phytoliths.