How to check whether or not property of class is set or not.?

public class clsListingDetails
{
        private string _SystemName;
        public string SystemName
        {
            get { return _SystemName; }
            set { _SystemName = value; }
        }

        private string _MaxCharacters;
        public string MaxCharacters
        {
            get { return _MaxCharacters; }
            set { _MaxCharacters = value; }
        }
}

public class User
{
public Dictionary<string, ArrayList> colListingDetails { get; set; }
}