Normally (specific) surface area is measured by BET (physisorption of nitrogen at its boiling point).  This is what would be considered an active surface area (for reactivity, dissolution, gas absorption etc).  The internal  'surface' is grain boundaries, as you indicate, which are basically fused chemically to each other and not usually releasable (one possible exception is stress corrosion milling).  Take a look at:

Nov 11th, 2008. Dispersion and nanotechnology  http://tinyurl.com/hpywsge

and March 27th, 2014 Nanotechnology characterization techniques

https://www.brainshark.com/malvern/vu?pi=zCTzgg96hz0z0

For micropore, or grain interface, Small-Angle X-ray Scattering (SAXS) might have a role to play.

Thank you both. So the understanding is that grain boundaries are usually not available to reactants (speaking in a context of heterogeneous catalysis) but could be active surface if the rupture is larger than reactants (e.g., small gas molecules or ions in aqueous solution).

@Xiangchen Huo  Generally speaking, yes you are correct, the grain boundaries are not normally or quickly accessible to reactant.  But they are a 'line of weakness'.  Take a look at slides 60 onwards in the first webinar I attached above:

Nov 11th, 2008. Dispersion and nanotechnology  http://tinyurl.com/hpywsge

Also these regions mean that certain atoms can diffuse very rapidly through these boundaries - for example, gold in lead.  Look at slide 21 in the following and then slides 41 onwards:

August 13th, 2015 The importance of the measurement of diffusion in 2-phase systems

https://www.brainshark.com/malvern/vu?pi=65736585&b=1&tx=120486&c1=22