Check for unsupported downsampling mode first ?

[centminmod]

I started playing with Guetzli adding it to my optimise-images.sh script but some jpg images return an error while others don't. The error is

Unsupported input JPEG file (e.g. unsupported downsampling mode).
Please provide the input image as a PNG file.

I assume that means the source jpg file isn't a high quality source ? But does Guetzli have a command that can quickly check a target image is of high quality or now ? How is Guetzli determining the check ?

thanks

George