Here is a way to measure it using the person's moment of inertia. Also, compared to many of the other answers to this question, this method is not only feasible, but actually reasonably safe for the subject.
1. Measure the mass $m$ of the person; the length $l_b$ and average cross-sectional width (from the front) $w_b$ of the person's body (excluding the head); and the length $l_h$ and average cross-sectional width $w_h$ of the head of the person whose head we wish to know the mass of.
2. Obtain a rigid stretcher that includes restraint straps, like this one:
3. 675
Securely mount the person whose head is to be measured onto the stretcher, using the included restraint straps, so that the 'front' of the person is facing upward.
4. Measure the moment of inertia $I_{gross}$ of the person in the stretcher about an axis extending upward through their neckn.
5. Measure the moment of inertia $I_{tare}$ of the stretcher (without the person in it) about the same axis.
Now compute the net moment of inertia: $$I = I_{net} = I_{gross} - I_{tare}$$
For simplicity, I will model the head and body of the subject as if they were thin rectangular plates rotated about the axis at one end.
Let $m_h$ be the mass of the head of the person, and $m_b$ be the mass of their body. We know, then that $m_h + m_b = m$, or in other words, that the masses of their head and body together add up to the mass of the entire person.
$$I = \frac{m_bh_b^2}{3} + \frac{m_bw_b^2}{12} + \frac{m_hl_h^2}{3} + \frac{m_hw_h^2}{12}$$
Solving this equation yields:
$$m_h = \frac{4 l_h^2 m+w_h^2 m+12 I}{4 h_b^2+w_b^2+4 l_h^2+w_h^2}$$
Going further:
If you wished, you could use a more accurate model of the person, resulting in a more complicated equation for their moment of inertia. Also, if one wished to obtain a more accurate measure, one could measure the person's moment of inertia about several different axes, in order to compensate for nonuniformities in the density of the body as a whole, rather than just differences in the density of the head and the rest of the body. 15