The Torah lists the Bat Ya'ana as a non kosher bird. It's true that the ostrich eats mostly plants but it will also happily eat insects, lizards and small tortoises. The feet configuration is not reliable. Kosher birds generally have three toes forward and one behind.
Ostriches have two toes pointing forward.
One very good guideline for kosher birds is the gizzard. The walls are highly muscular and can be easily peeled out. These birds swallow grit which is used to grind the seed in the gizzard. Non kosher birds also have gizzards, but not with thick muscular walls. I don't know about ostrich gizzards, but even if the walls are thick, that would not prove kosher status.