In order to be classified "countertenor", your voice has to not only have to have the range but the lighter colored tone production of a countertenor. As
this page states, a countertenor is able to produce a full-bodied tone with their falsetto, and it has a certain quality to it unlike men who are tenors or basses. Further, this page states that singing as a countertenor is a decision, because you've decided to embrace singing in your falsetto and training that range over your lower voice. (This page is awesome, I'd read it.)
Put simply, most men can sing in falsetto, and most men can sing kind of low--some of these men are baritones, and some of them are countertenors, and some of them are whathaveyous, even though they have similar ranges. Terms like bass, countertenor, and baritone are not terms of range, but terms of tone color. The difference between a mezzo soprano and a soprano is not necessarily the range but the tone quality.
So, what does your tone sound like in the upper range? To get the idea of a countertenor's sound, I'd suggest listening to groups like The King's Singers or Chanticleer...they have fantastic singers.