how do you know?
I look at the diameter of other people's ponytails right at the tie. Mine is 2-3 times as thick as other people's. That's how I know.
why do you care if you have thick hair or not?
If you have THICK hair...you KNOW it.
Thick hair can be the toughest hair to grow out, giving you lots of trouble.
I know, because I have very thick hair.
Well theres 2 kinds of thick, theres wavy/curly thick and straight thick, the thickest is straight thick because the shape of the hairshaft is round as opposed to oval like wavy thick, thats why people with seemingly really thick curly hair can only make smaller pony tails, but when its loose it fluffs out like crazy, where as those with straight hair tie it back and make quarter size and larger tails but when they let it down it doesnt look thick at all. Hopefully that made some sense.
If no one can show you then after awhile you will get the feel for knowing the difference.
The easiest way to tell is when strangers or your hairdresser tell you that you have thick hair. People seem to like to point that out especially when they don't have thick hair themselves.