Unfortunately, Serato DJ does not have a dedicated song pre-listen/preview feature - you must first load a song into a deck or a sampler. Most other DJ software has it, and if you've ever used it, you know how handy it is, especially after songs are already loaded in the decks.

Many people have asked (in the Features Suggestion Forum) for Serato to add this feature - to date it has fallen on deaf ears. What follows is my solution to the problem. It is specific to Windows, although I'm sure it can be altered for Mac's. It requires an additional piece of hardware, two cables, and a different default Windows Player. Here goes -

Serato has a keyboard shortcut - Ctrl O - that plays a song highlighted in it's library browser through the laptops headphone jack, using Windows Media Player (or whatever default player you have). We will take advantage of that, but first the default player must be changed to this -
@MAX Tray Player - Play your favorite music right from taskbar

Why? Because it is one of the few players that runs completely in the background and never captures "screen focus". I'm going to assume you know how to change your Windows default player to this new one.

Now, you will need a headphone mixer. Anything should work as long as it's stereo - I already had a Multimixer 6 (typically around $30 on ebay). Next, a 1/8" stereo jack and cable to feed audio from the laptops headphone jack to the mixer, then a 1/4" stereo jack and cable to feed audio from your controllers headphone jack to the mixer. Your DJ headphones also plug into the mixer. At this point it should be self evident how this works.

Turn off the PL button(s) on your controller, hit "Ctrl O" on your laptop, and using the laptops up down arrows you can fly through your library pre-listening! You can also customize keyboard shortcuts for the Max Tray Player - just make sure they don't conflict with any of Serato's. That's it.