One of Photoshop's relatively new tools is the "loupe". This handy previewing tool lets you make judgment calls about one or more photos from the Bridge, without actually opening the pictures in the editor. The loupe technique is faster, and can avoid a lot of hassle with juggling windows and changing zoom levels.
You access the Loupe by clicking on an image in the Preview pane in the Bridge. A rectangular window appears showing a 100% zoom of a small part of the picture. You can drag the window (based on the idea of a jeweller's loupe, a one-eyed magnifying glass) around to examine the most important area of the photo.
One corner of the loupe bulges out, almost like an arrow from a cartoon speech bubble. The arrow points to the spot in the photo that's being magnified, so you can see the scaled-down region and the magnified version at the same time. If the loupe blocks part of the photo you want to see, drag it to the left or right to flip the loupe so the arrow comes out of a different corner.
You can use the magnification to judge whether a photo is worth saving, cropping, or blowing up. For example, you can quickly check whether a wildlife shot has enough detail to be worth cropping and showing at close to original size. You can look at some small detail, like a license plate or sign, to see if it's legible at full size.
Where the loupe really comes in handy is to compare multiple photos. Selecting multiple pictures in the Bridge displays several seriously scaled-down thumbnails in the Preview area. If you took several pictures in quick succession, for example an action sequence where some shots are blurry, or a group photograph where different people have eyes open or closed, picking the ideal one can normally be time-consuming. Put them all up in the Preview pane, open a loupe on each one pointing to the same spot, and quickly decide which ones are too blurry, which ones are too shaded, which one has the best detail for the important region, and so on.
In this example, I put a loupe on 2 photos taken a couple of seconds apart, to see whether the shadows on the face in the first photo are too bad.