Using your regular shampoo and conditioner regimen, wash your hair as you normally would. Rinse your hair clean of shampoo and conditioner. Make sure all chemicals and products are rinsed thoroughly out of your hair. Comb through your hair first and then gently squeeze your hair using a clean t-shirt or microfiber towel. Do not dry it too much, or else you may not be able to determine the porosity, or how well your hair retains moisture, very well. Take chunks of hair in your hands and feel them from the roots down to the ends. Gently squeeze your hair to feel the moisture.   Low porosity: If your hair feels almost dry, your hair has not retained much moisture and you have low porosity.   Medium porosity: If your hair is pretty wet but not sticky, then your hair retains a medium amount of moisture and you have medium porosity.  High porosity: If your hair feels sticky, as though the water will take a long time to leave your hair, then you have high porosity. Your hair retains a lot of moisture. Pull out a strand of hair and float it in a large bowl of water. The bowl needs to be large enough so that the hair strand will not touch the sides. Observe what happens with the strand of hair.  Low porosity: If the strand floats and does not sink at all, you have low porosity.  Medium porosity: A strand that sinks after a while will indicate medium porosity.  High porosity: If the strand sinks to the bottom of the bowl quickly, you have high porosity. The weather may affect your hair; if it is very humid, for example, your hair may react slightly differently than on a drier day.

TL;DR in Vietnamese:
Gội đầu. Dùng khăn lau tóc cho khô tương đối. Dùng tay vuốt tóc. Thả tóc vào một bát nước lớn. Thử lại vào một ngày khác.