When no OAuth token is present there's this err:

{
"error": "Unauthorized",
"status": 401,
"message": "OAuth token is missing"
}

But a missing client-id doesn't return an error that it is missing, instead it responds with:
{
"error": "Unauthorized",
"status": 401,
"message": "Client ID and OAuth token do not match"
}
(which is the same error if both are sent, but don't match).