Q:

What would a good HTTP response to show existence of an entity?

I am implementing an API and I want to return an indicator that an entity exists, say for example, it is to check if an order exists in the backend....
GET    /order/:order-ref   

I could return a JSON payload like this:
{
  true
}

but it seems heavy. I was thinking of just returning a HTTP status to indicate the same. What HTTP status is correct in this case?

A:

It seems heavy.

As CodeCaster pointed out, that's not a valid JSON and I don't see how it can be considered heavy. 

I was thinking of just returning a HTTP status to indicate the same.

Depending on your needs, you could use the HEAD method. It's identical to GET requests, except that the response will contain only the status line and the headers, without any payload.
HEAD /orders/1 HTTP/1.1
Host: example.org

Quoting the RFC 7231, the document that defines the semantics and content of the HTTP/1.1 protocol:

4.3.2.  HEAD
The HEAD method is identical to GET except that the server MUST NOT send a message body in the response (i.e., the response terminates at the end of the header section). [...]

What HTTP status is correct in this case?

If the resource exists, return 200. Otherwise return 404.
HTTP/1.1 200 OK
Date: Tue, 8 Oct 2019 13:39:14 GMT

HTTP/1.1 404 Not Found
Date: Tue, 8 Oct 2019 13:39:14 GMT

