If you want to know easily the void fraction of a porous material you can put it in a note quantity of water and calculate the volume of the water adsorbed in the material. I think this is a good solution.
There are several practical methods for measuring voids fraction, e.g. try to use BET test. Moreover, you can also perform another method, measure the density of your samples experimentally and theoricaly, thence compare their volume and weight with each other. Differences between theroricaly can experimentally calcuated volume based on density can determine the exact voids volume or fraction within the structure.