[01100011]

You could put the whole thing in a faraday cage and look at the emissions with a spectrum analyzer. There's probably a bunch of wide spectrum noise though, thanks to all those squarewaves running at various frequencies. If you were smart, you might try modulating one of those signals(sort of like the 'spread-spectrum' feature of many BIOSes, but with information doing the modulation and not just noise). You might be able to sneak other signals in amongst all the noise. Hell, spread spectrum signals can sit below the noise floor... I don't really think there's a way you could be 100% certain that the laptop wasn't exfiltrating data.

[meredydd]

Transmitting radio waves with a spread-spectrum CPU clock, you say?

Here's a project to turn the Raspberry Pi into an FM radio transmitter using this exact trick:

http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspb...