When opening a file, holding the Option (Mac) / Alt (Win) + Shift will open a flattened version of a file (instead of a layered one) if the original layered file was saved with the Maximize Backwards compatibility preference turned on.
Is there a way to tell if an image has been saved with Maximize Compatibility without opening the image? via Metadata or similar?
I don’t think that there is an easy way to tell. Although any application that needs the flattened version to render a preview will be unhappy with the file. I discovered that many of my early work was not saved this way when I tried importing the files into Lightroom.