The easiest way to find out your hair porosity is to get a glass of water and put a clean strand of hair into the glass.
Let it sit for about 5 minutes.
If your hair strand floats, you have low porosity hair. Having low porosity hair simply means that your hair has a hard time allowing moisture in and allowing moisture to leave. If your hair strand sinks, you have high porosity hair. Having high porosity hair means that your hair has no problem allowing moisture in, however the moisture leaves almost as fast as it enters.
If it floats for a while and then sinks, I would still consider your hair to be high porosity.