When you print documents with your Mac, they show up in your printer's queue. You can view that queue by clicking the printer icon in your Dock. Sometimes, though, it's difficult to tell exactly which documents are which in that queue—especially if you're printing a bunch of untitled or similarly-named files. Hints reader ppinheiro76 figured out a delightfully simple way to get quick previews of the files you're printing.
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?