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

Summer Products /Routine:
Clarifying: ACV rinses, Suave Professional Men Deep Clean Peppermint
Low Poo: Renpure Luxurious Argan Oil
Co-washes: VO5 Kiwi Lime Squeeze or Strawberries & Cream
RO: VO5 KLS or S&C, Cure Care
LI (if needed): VO5 S&C, VO5 SC, Cure Care
Stylers: FSG, AIF,
Tresemme Flawless Curls Bouncy Curls gel
PT: IAGirl's (link) as needed