How do you set keymap item properties?

import bpy
wm = bpy.context.window_manager
km ='Object Mode')
kmi ='object.origin_set', 'C', 'PRESS', alt=True) = 'GEOMETRY_ORIGIN'
# -->['type'] = 'GEOMETRY_ORIGIN'

In some operators the properties can be accessed as variables. In some others properties that exist in side operators, only as dictionaries (the commented line).

How do you detect how to access the properties?
I use the try-except blocks but it does not work. Any other ideas?