'Run the system checks on all ModelAdmins, except if they aren\'t
customized at all.'

def check(self, app_configs):

    if (app_configs is None):
        app_configs = apps.get_app_configs()
    app_configs = set(app_configs)
    errors = []
    modeladmins = (o for o in self._registry.values() if (o.__class__ is not ModelAdmin))
    for modeladmin in modeladmins:
        if (modeladmin.model._meta.app_config in app_configs):
            errors.extend(modeladmin.check())
    return errors
