This will check a single characters Accessory slots for item #81908

CheckCharacterAccessory

using Newtonsoft.Json.Linq;
using System;
using System.Linq;
using System.Net;
using System.Text;

namespace CheckCharacterAccessory
{
    class Program
    {
        private const string ApiKey = "PASTE YOUR API KEY HERE";
        private static readonly WebClient HttpClient = new WebClient { Encoding = Encoding.UTF8 };

        static void Main(string[] args)
        {
            var itemId = 81908;
            var characterName = "PASTE YOUR CHARACTER NAME HERE";

            Console.WriteLine($"Item {itemId} is in Accessory slot #1?  {CheckAccessory(characterName, "Accessory1", itemId)}");
            Console.WriteLine($"Item {itemId} is in Accessory slot #2?  {CheckAccessory(characterName, "Accessory2", itemId)}");

            Console.ReadLine();
        }
        private static bool CheckAccessory(string characterName, string slot, int itemId)
        {
            var url = $"https://api.guildwars2.com/v2/characters/{characterName}?access_token={ApiKey}";
            var json = HttpClient.DownloadString(url);
            var character = JObject.Parse(json);

            var accessory = from equipment in character["equipment"]
                            where equipment.HasValues
                            where (string)equipment["slot"] == slot
                            select (int)equipment["id"];

            var isInAccessory = false;

            foreach (var item in accessory)
            {
                if (item == itemId)
                {
                    isInAccessory = true;
                }
            }

            return isInAccessory;
        }
    }
}