with a classical flute you didn't need to blow at all - just put your ear against the "blow hole" and finger the notes - you could hear it....... don't know how well that will work on a keyless flute though

Works just great on an open-hole flute thanks, been doing it for many years as a late-night practice technique.

However my variation on the technique is to put the flute in normal playing position against the bottom lip and then open my mouth slightly - the increated resonance helps the notes form a very quiet sound, which is then transmitted through the bones of your head to your ears.   

Don't think anyone has also mentioned that you can practise tin whistle nice & quietly by holding it in flute position and blowing sideways across the fipple.