A full amplitude test tone and an oscilloscope is best for this purpose as it lets you find the clipping point of your laptop's output. You'd want to stay in the upper range of your laptop's output capacity before clipping as long as it does not make the amplifier clip. What you want to do is to optimize your gain structure to minimize the noise floor.
Of course you can just play with the volume control to listen for audible clipping then back off to establish a rough "safe zone."
To do the latter: Set your iTunes volume to max (looping playback of a loud passage), dial a safe gain on your amplifier, then start playing with the PC master volume control. Dial it back once you detect harshness -- your new setting should be your PC master volume control level for maximum dynamic range. Adjust the amplifier gain until you hear distortion, then dial back to a safe level. After that, you can start playing your music and use iTunes as your volume control knowing that you have somewhat optimized your signalling level (resulting in better SNR). You can use some form of peak normalization if you want to always have max amplitude available to you.
