How to measure power absorbed by an object in HFSS and FEKO?

The equation for SAR = (σ/2ρ) |E|2 (W/kg)

Basically SAR is the amount of Energy absorbed by a dielectric body mass.

There are bunch of solvers that simulate SAR. Could you shed some light on how they do it, meaning how do they calculate the density and mass of the object? How do they calculate the E field? Is it the S11 reflected back? How do they calculate σ for the whole object which may be inhomogeneous as the human head?

Now can SAR be measured in a real life enviroment? Say I want to measure the SAR in the human head from an antenna placed 1cm away. How do I determine the σ, ρ, and E to find the measured SAR?

I would really appreciate an answer to this topic since I am basing my research on it and would like some leads.

Thank you.