Its really a genetic thing. Your hair thick or thinness depends on the amount of hair follicles you have per square inch of scalp. If you a natural its easy to have thickness regardless of having thin or thick hair. But if you are relaxed, you will be able to tell immediately if you have thick or thin hair.
To determine the quantity of hair you have, pull your hair back into a ponytail. If it's very slim—about the diameter of a dime, you have a thin hair type. Another clue: can you see your scalp when your hair is wet? If so, your hair is probably a thin hair type.
If your ponytail is "two-fisted"—and looks almost like a true horse's tail, your hair a thick hair type. When your hair is wet, it covers your scalp very well, without any scalp showing through.
If you fall between these extremes, you have a medium hair type—the most common hair type.
Thin hair type
Your hair type has very, very small in diameter strands, you'll tend to be weightless and have flyaway hair. Frequently, it's straight hair and won't hold a curl easily. Your hair can be maximized with blunt hair cuts that capitalize on its often silky hair texture. Hair products that reinforce the hair—proteinized conditioners, light gels, and mousses—can also boost thin hair's "wimpy" looks. Your hair has to be permed with extreme care to prevent damage.
Medium Hair Type
Your hair has the widest range of possibilities. It's the most common and the most manageable.
Thick Hair Type
Your hair type is often very strong and you're likely to have hard to manage hair. A layering haircut can give thick hair more shape and direction. Silkening products—pomades, surfacers, and even strong hair gels can add control. Very straight, thick hair is often resistant to perms, can be stubborn about holding a curl, and likely to need frequent perming to maintain hair's new texture.