Preview the print queue with Quick Look | Printers | Mac OS X Hints | Macworld: Once you're in the print queue, just double-click on an individual print job to open a Quick Look preview of the document being printed; since it's Quick Look, you could instead tap the spacebar to trigger the same effect.
This reminds me that Quick Look pops up in some unexpected places, and it's often a delightful surprise. For example, I frequently use it in Open and Save dialogs to preview files there. And don't forget that the print queue has some other hidden functionality, too: Remember the one about dragging documents directly into the queue to print them?