Determine if opposing pieces are captivated

Hello everyone. I am trying to write a simple application that resembles the game "Computer Go", but I am stuck with this problem: If the board is represented by a dimensional array, with which algorithm could I tell if one player's pieces are captivated by the other's? In other words, how is it possible to recognise whether the pieces of one colour form a circle?
Sorry for my English.

Is there any source code of the game, bacause I cannot find any?