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

Autumn Products /Routine:
Clarifying: Herbal Essences Tealightfully Clean, Suave Professional Deep Clean Peppermint
Low Poo: Renpure Luxurious Argan Oil
Co-washes: VO5 Kiwi Lime Squeeze or Strawberries & Cream
Tresemme Expert Selection Perfectly (un)Done, Cure Care, VO5 KLS
LI (if needed):
Tresemme Perfectly (un)Done, VO5 KLS, Cure Care
Stylers: FSG, L'Oreal Clean gel, Tresemme Flawless Curls Bouncy Curls, LALSG, HETT, AIF
PT: IAGirl's as needed