Hi Sean,
The simple answer is you will be able to tell you're playing a high resolution track when the light on the front of your Rega DAC tells you that a 96kHz signal is being received. How you get to that stage is a little more difficult!
As has already been explained the Apple TV2 has a set output of 48kHz, so won't help and although its hard to find exact technical specs for the Airport Express it too is limited to 44.1kHz as far as I know and I can find out.
The Rega can accept upto 24 bit 192kHz signals through its Coaxial or Optical inputs but it limited to upto 16 bit 48kHz through its USB input.
As has been suggested if you don't have an optical out from your laptop the Musical Fidelity V-Link or the m2tech Hiface are two options of turning your laptop's USB port into either a Coaxial of Optical output which can then in turn be linked to the Rega and will be suitable for playing Hi-Res music files. The V-Link is limited to 24 bit 96kHz whereas the M2Tech can output upto 24bit 192kHz. There are I'm sure others products capable of this task but I haven't come across them.
Now that was the easy bit....
Mediamonkey needs an ASIO plugin to be capable of outputting bit perfect high resolution audio, I haven't experimented with it much but effectively it will allow the player to output the source music at whatever resolution it was made in or purchased in.
I have however tried another free media player called Foobar and together with the WASAPI plugin am able to play high resolution music at its native bit-rate. This post would go on for some time if I tried to explain how to do both of these but PJPro has done some excellent threads on Foobar and if you google Mediamonkey and ASIO I'm sure it will explain it much better than I could.
Itunes for windows is allegedly supposed to offer bit-perfect playback but having tried it at best you have to change the computers audio output settings everytime you change from a normal 16/44.1 ripped CD to a downloaded 24/48 or 24/96 track.
Good Luck, I also found some of the FAQ's on Computer Audiophile's website very helpful in understanding all of the above!