How can you tell whether or not an image (or image file) has been edited?

That would depend on Photoshop leaving traces that it made modification.  If Meta data has been striped was it striped by Photoshop Save for web  or by some other application. If there is just a pixels layer how can you tell with a photoshop script that the image has been modified and that the modification was made with Photoshop?   There are programs that examine images and try to identify images that have been modified. I do not know if any that finds an image has been edited can identify what application modified the image.

There is also the DocumentAncestors section, which is a full record of Paste and Place operations. I've no idea how to read it, but it's there for what I assume are forensic purposes.

And to see this metadata, go to File > File info > Photoshop.

If History log was enabled when the file was saved, it will contain metadata about the edits.

The settings below will provide the most detailed information.