Is Photoshop able to determine whether a particular image has previously been modified in some way?
That question seems more than a little problematic.
If you want to limit it to whether a digital photograph has been edited after shooting that would limit it somewhat, but I suspect some changes can be nearly impossible to detect.
But Photoshop is image editing software and therefore not really concerned such image forensics, I guess.
As C. Pfaffenbichler said, that's not really PS' domain. It's easy enough to detect crude changes in a JPEG image because or compression artifacts and likewise it is easy to detect lazy jobs with the clone stamp, but anything beyond that requires dedicated tools and in addition quite some understanding of the underlying technical aspects. A lot forensic stuff is based on using specific formulas or transforming the image to other presentations of the data and it requires to be able to interpret these abstracted things to determine forgery or manipulation...
EXIF viewers like Opanda will often tell you when an image was taken, and last edited, and even what software was used. It won't tell you what, and how much has been done to an image though.
Thanks for the responses guys. I'm not looking for an in depth analysis of an image. It's enough for me to know whether a digital image has been modified in any way by any piece of software. Will an EXIF viewer like Opanda be able to tell me that?
Thanks to C. Pfaffenbichler for the links as well. Fourandsix looks very promising, but if what I'm looking for can be done with an EXIF viewer, that would be ideal.
The short answer is NO, you cannot tell for certain. No digital image will be admissible in court, for example, unless chain of custody can be proven.
Programs like "FourAndSix" do things like extract embedded JPEGs, but that process could be fooled.
It's not hard to imagine someone knowledgeable and working with a purpose being able to "spoof" all the right data so that even a raw image from a camera could look unmodified, but be completely bogus.
So a relevant question might be what the original poster is up against … can the people from whom the images come be expected to be that knowledgable or not?
And what is that expectation worth?
As others have mentioned, that is not in the domain of PS, but here is another program, that does a fair job (though not perfect in my observations) of just what you are asking for - ThumbsPlus by Cerious Software. It is a cataloging program, not unlike Bridge, but with some extra powers. I use it, along with Bridge.
It can detect a lot of potential manipulation, and will flag all such Images with a little i icon. I know that it is not perfect, as it has missed some Images, that I had manipulated, but it's still pretty good. I also have not upgraded from ThumbsPlus 8 Pro, so do not know how the newer versions might work.
PS - what it does not, or at least did not, tell you, is what manipulation was done.