Hey, I figured out how to tell.....
Just open the image in photoshop, and then go to "save as", and the menu will automatically be set on the type of jpeg it is, before you actually save it. Then you can just cancel and close the menu. I tried it several times with existing images, and even made a few new images using each type, and then re-opened them, and it detected it correctly every time.
There might be an easier way to tell, instead of going thru the motions of almost
re-saving, but at least this does work, and it's easy.