In Acrobat Pro, open the Output Preview panel and select the Object Inspector.
You can then click on any element in the PDF, including, among other things, images, and you will get info about the selected element.
In clicking on an image you can get the resolution, profile if any, and other information.
As mentioned about, opening a PDF in Photoshop will rasterize everything in the PDF at the same resolution that you selected in the Open PDF dialog.
This will tell you NOTHING about the original resolution of any image. and again will rasterize ALL vector elements, including text.