Photoshop - verifying true resolution - how to find out if file has been resampled

There are some photographic exhibitions and salons that demand the original file, and some that only allow a limited amount of cropping.  (Usually Natural History — Nature — categories).  If the psd file has photographic content, then you'd know the sensor size of the camera used to take it, but obviously not how much that content had been cropped.

The bootom line here is that with the quality of current lenses and cameras, you'd expect a certain degree of quality, and that would either be present, or it wouldn't.  Or you could take a really sharp image of your own, and up-res and up-res it again, inspecting the quality loss each time, and use that as a datum to judge said psd file.  You also need to take into acount the new Super Resolution feature in Camera RAW.

While it is true that resampling will introduce artifacts what really matters is the final image. So, I would approach this differently. On receiving such an image, rather than worrying about the file history, whether it was resampled up or down, just look at the image. Either the quality will be good for printing or it will not. I've seen more images that I would reject for other reasons, focus - exposure - noise - camera shake etc than I would for resampling.

Dave

You are mentioning resolution and print size without any mention of pixels that are most important when working in Photoshop. What matters most is number of pixels not resolution pixel per inch.

It seems you are looling at print size in cm or inches when ticking Resample and changing resolution pixel per inch. What you do not see is change in number of pixels in background. That is most important because if Ps does not have pixels it must construct them from nothing, just by looking at image and it must do that in seconds. ALgorythms are better and better but... Image will look blurry when upsampled as alreday stated above. 

"If the image size is changed to match 300 dpi with resample ticked then the resolution does not change, only the physical file size."

Maybe I do not understand well what are you talking about but still... The most important thing is how many pixels are there. I do not think you can track changes in file dimensions using metadata that ships with PSD file although I actually never though about that.

If you want to track such changes then you may be able using Preferences > History Log to Metadata. Edit Log Items > Detailed should work. Ask creator or sender of files to check that option before start to working on file.

The true resolution is the number of pixels.

The ppi number is just metadata, it's unimportant in itself. You can change that arbitrarily without changing the actual data.

Resampling the pixel data will always cause severe quality loss. That's how you can tell.