You can calculate the theoretical surface area of particles based on their size, assuming that they are spherical and non-porous. And of course, you can do the reverse thing, i.e., predicting the particle size from the measured surface area, see Eq. (1) in the attached paper.

If the measured surface area is significantly higher than the theoretical value, then you can conclude that the particles are porous. But predicting the value of porosity from the surface area would require to know the pore-size distribution.

Alain