Angels, Struttswife explains about how to judge porosity and describes a "strand test" you can use to evaluate it, here. For me at least, that test was spot on, and confirmed the other signs I was seeing that said "low porosity".

If you want a really definite answer, you can spend $10 and get a hair analysis. It will tell you exactly what your porosity, elasticity, and texture are.
CG since 11/2011
2c /3a F /Mii, low porosity

Winter Products /Routine:
Clarifying: Herbal Essences Tealightfully Clean
Low Poo: Renpure Luxurious Argan Oil
Co-washes: Suave Naturals Tropical Coconut or Daily Clarifying
RO:
Tresemme Expert Selection Perfectly (un)Done, Cure Care, Suave Tropical Coconut or Daily Clarifying
LI (if needed):
Tresemme Perfectly (un)Done, Renpure Brazilian Keratin Straightening (old formula), Cure Care
Stylers: HETT, AIF
PT: IAGirl's as needed