Not 100% (is that a pun?) sure, but when you view "actual pixels" in Photoshop, the viewing window is huge and says 100%. I note that some people cut out a chunk of this and display it on forums to either prove or disprove arcane points in endless debates.
Use the Nav tool, increase image to 100%.
Cut out a "chunk" of it , keep it at 100% (1:1 resolution) and one will have a 100% cropped photo.
Where I was confused: I would use the cropping tool. End up with an image (i.e. 4X6), then blow that up to 100%. Instead of having a 1:1 resolution, I must have been 4 or 5 to 1, making even the clearest crop grainy.