Three things can tell you about your hair type: its Structure, its Texture and its Condition. After a shower, let your hair dry on its own, and then take a closer look at it!
When talking about Structure we mean straight, wavy, curly or afro. Straight hair is naturally smooth and shiny when lying flat. Wavy hair has more body and a natural "kink" to it. Curly hair is exactly that, curly! And Afro hair has really tight curls and lots of volume.
Hair can be fine, medium or coarse texture. Holding a single strand of fine hair, you won't really feel it rubbing between your fingers, and barely see it if you hold it up in front of your eyes. Medium hair is visible and easy to feel between your fingers. Thick or coarse hair is easy to see and can feel wiry between fingers. Tied up in a ponytail, fine hair has a circumference at the ponytail of less than 2 inches, medium hair is 2 to 4 inches, while thick hair would be over 4 inches around. Try it out and see!
Your hair's condition is oily, dry or normal. Oily hair can look greasy within a couple of days of shampooing, though dry shampoo is now a great daily solution for this. Curly hair is often dry, feeling rough and needing moisture to make it feel soft and smooth. If you don't experience any of that, you have normal hair. You still need to maintain it with a gentle shampoo and minimal heat styling.
Now you can diagnose your own hair type and what products would work best for you!