There are two methods you can use to find out how porous your hair is.
The Float Test: Take a couple of strands of hair from your comb or brush and drop them into a bowl of water. Let them sit for 2-4 minutes. If your hair floats, you have low porosity. If it sinks, you have high porosity.
The Slip'n'Slide Test: Take a strand of hair and slide your fingers up the shaft (toward the scalp). If you feel little bumps along the way, this means that your cuticle is lifted and that you have high porosity. If your fingers slip smoothly, then you have low porosity hair.
Hair with low porosity has a tightly bound cuticle layer with overlapping scales that lay flat. This type of hair is usually considered healthy, and is often very shiny, especially when it's dark in color. Low porosity hair repels moisture when you try to wet it and is hard to process since it resists penetration of chemicals.
Low porosity hair is also prone to build-up from protein-rich deep conditioning products, which can leave it feeling stiff and straw-like. Stick to protein-free, daily conditioners with humectants such as glycerin or honey. Use moderate heat with protein-free deep conditioning treatments to help open up the tightly bound cuticle.
Low porosity hair requires moisturizers rich in emollients such as shea butter, jojoba oil, coconut oil and mineral oil. It also benefits from humectant products, which attract and hold moisture to your hair.
High porosity can be either an inherent property of hair or the result of damage from chemical processing, rough treatment or environmental damage. High porosity hair has gaps and holes in the cuticle, which let too much moisture into your hair and leave it prone to frizz and tangling in humid weather. Even simple acts such as bathing, swimming and shampooing can create more damage and breakage due to the sheer amount of moisture highly porous hair can absorb.
Be sure to use anti-humectants in climates with high heat and humidity. This will help seal your damaged cuticles and prevent them from absorbing excess moisture in the air.
Because highly porous hair can also lose moisture easily, it's important to use leave-in conditioners, moisturizers and sealers. Layering these products will help your hair hold on to the moisture you're giving it. You can even follow up with a heavy hair butter to help fill the gaps in your damaged cuticles and further protect your hair from losing too much moisture.