K this one I donno how to explain, must call in the vocal tech elites already... haha..
One way I use to determine whether I'm singing in the head voice is that the resonance is focused in your face, around the upper sinus cavities.
Try singing low to high, again, but using the word "Hee". So go "hheeeeeeEEEE" all the way. Pay close attention to where the resonance shifts to when you go higher; Head voices are usually and generally used to sing high pitches so you should be able to sense that section when you go higher. Try it?