Check if the response of a get api is a proto object

I fetch a proto object from an api , is there anyway I can validate that the response is a proto object.

One thing which I did was check content-type as mentioned in the json counterpart of this question. But the answer goes on further to validate json using JSON.parse

The content-type of a proto file comes out to be application/octet-stream ( protobuf can have this content-type , check this )

Is there any other check I should do to validate response is a proto object ?

EDIT : Proto means Protocol Buffers : https://developers.google.com/protocol-buffers