This would only check for the dirty flag on the project object (it would be true if you modify project settings for example). You also need to check for the dirty flags on all work units.
Here is how:
objects = client.call("ak.wwise.core.object.get", {"from": {"ofType": ["Project", "WorkUnit"]},
"options": {"return": ["workunit:isDirty"]}})["return"]
_project_is_dirty = any(map(lambda x:x["workunit:isDirty"],objects))
pprint.pprint(_project_is_dirty)