It may be obvious to those in the know, but photographers who are new to digital sometimes think that viewing an image at 100% means seeing the whole image (100% of it) on the monitor screen. Although entirely logical, this is very much wide of the mark.
100% means that, on a monitor screen, one screen pixel is displaying one image pixel. So, a more accurate description would be 1:1. This is the only way to see every pixel in an image.
In Photoshop, double click the Zoom Tool or use the keyboard shortcut Cmnd+Alt+0 (Mac); Ctrl+Alt+0 (PC). Confirm the title bar of the image reads 100%.
An image should be able to withstand scrutiny when viewed at 100%, but do bear in mind that this is a very tough test. It's a bit like taking a microscope to a negative or transparency. Any minor flaws that are visible at 100% will probably not be visible in print. However, a client will almost certainly inspect any image you sell them at 100% and expect it to be faultless. So make sure your images look OK at 100% - and always carry out any retouching/post production work at 100%.