When Image Transcoding is enabled and SmartVision is enabled, and an image is successfully transcoded, the response header will contain an X-Instart-Streaming header such as this:
X-Instart-Streaming: stranscode; 70
The number at the end is the quality level that was determined by SmartVision and applied to the transcoded image; in this example, its image quality was set to 70%.
If Image Transcoding is disabled, or is enabled but SmartVision is disabled, or if the image is a PNG or has been transcoded to WebP or JPEG XR, this header will not be present. In those other cases, if transcoding has occurred, the only visible change in the client side will be a reduction in size compared to the original image.
If your web property is supposed to be configured to enable Image Transcoding with SmartVision and you don't see this header, the configuration might be incorrect. Please contact email@example.com if this is the case.