Someone

int checkBoard(char board[3][3]) {
    /* Across */
    if (board[0][0] == board[0][1] && board[0][1] == board[0][2] && board[0][0] != ' ') {
        if (board[0][0] == 'X') return 0;
        else return 1;
    }
    if (board[1][0] == board[1][1] && board[1][1] == board[1][2] && board[1][0] != ' ') {
        if (board[1][0] == 'X') return 0;
        else return 1;
    }
    if (board[2][0] == board[2][1] && board[2][1] == board[2][2] && board[2][0] != ' ') {
        if (board[1][0] == 'X') return 0;
        else return 1;
    }
    /* Down */
    if (board[0][0] == board[1][0] && board[1][0] == board[2][0] && board[0][0] != ' ') {
        if (board[0][0] == 'X') return 0;
        else return 1;
    }
    if (board[0][1] == board[1][1] && board[1][1] == board[2][1] && board[0][1] != ' ') {
        if (board[0][1] == 'X') return 0;
        else return 1;
    }
    if (board[0][2] == board[1][2] && board[1][2] == board[2][2] && board[0][2] != ' ') {
        if (board[0][2] == 'X') return 0;
        else return 1;
    }

    /* Diagonal */
    if (board[0][0] == board[1][1] && board[1][1] == board[2][2] && board[0][0] != ' ') {
        if (board[0][0] == 'X') return 0;
        else return 1;
    }
    if (board[2][0] == board[1][1] && board[1][1] == board[0][2] && board[2][0] != ' ') {
        if (board[2][0] == 'X') return 0;
        else return 1;
    }
    /* No winner */
    return -1;
}