Return a JSON string explicitly from Asp.net WEBAPI?

There are a few alternatives. The simplest one is to have your method return a HttpResponseMessage, and create that response with a StringContent based on your string, something similar to the code below:

public HttpResponseMessage Get()
{
    string yourJson = GetJsonFromSomewhere();
    var response = this.Request.CreateResponse(HttpStatusCode.OK);
    response.Content = new StringContent(yourJson, Encoding.UTF8, "application/json");
    return response;
}

And checking null or empty JSON string

public HttpResponseMessage Get()
{
    string yourJson = GetJsonFromSomewhere();
    if (!string.IsNullOrEmpty(yourJson))
    {
        var response = this.Request.CreateResponse(HttpStatusCode.OK);
        response.Content = new StringContent(yourJson, Encoding.UTF8, "application/json");
        return response;
    }
    throw new HttpResponseException(HttpStatusCode.NotFound);
}

Here is @carlosfigueira's solution adapted to use the IHttpActionResult Interface that was introduced with WebApi2:

public IHttpActionResult Get()
{
    string yourJson = GetJsonFromSomewhere();
    if (string.IsNullOrEmpty(yourJson)){
        return NotFound();
    }
    var response = this.Request.CreateResponse(HttpStatusCode.OK);
    response.Content = new StringContent(yourJson, Encoding.UTF8, "application/json");
    return ResponseMessage(response);
}