using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string stringJson = "[{\"tipo\":\"simnao\",\"pergunta\":\"rwerw#erwerewr ?\",\"tag\":\"#acimadopeso\",\"resposta\":\"\",\"$$hashkey\":\"object:410\"},{\"pergunta\":\"werwerwerwer ?\",\"tag\":\"#diabetes\",\"resposta\":\"\",\"$$hashkey\":\"object:412\"},{\"pergunta\":\"werwerwer ?\",\"tag\":\"#idoso\",\"resposta\":\"\",\"$$hashkey\":\"object:414\"}]";

        //Separa a string por aspas 

        string[] arrayString = stringJson.Split('\"');

        //Procura pelas strings que começam com "#"

        var listString = arrayString.Where(x => x.FirstOrDefault() == '#').ToList();

        //Mostra o resultado

        listString.ForEach(i => Console.Write("{0}\n", i));

    }

}