RelatedField class

using System;
using Microsoft.SharePoint.Client;

namespace Microsoft.SDK.SharePointFoundation.Samples
{
    class RelatedFieldExample
    {
        static void Main()
        {
            string siteUrl = "http://MyServer/sites/MySiteCollection";

            ClientContext clientContext = new ClientContext(siteUrl);
            Web site = clientContext.Web;

            List targetList = site.Lists.GetByTitle("Tasks");

            // Get a collection of lookup fields that use this list as a data source 
            // and that have FieldLookup.IsRelationship set to true.
            RelatedFieldCollection collRelatedField = targetList.GetRelatedFields();

            clientContext.Load(collRelatedField);
            clientContext.ExecuteQuery();

            foreach (RelatedField relField in collRelatedField)
               Console.WriteLine("Related Field ID: {0}", relField.FieldId);
        }
    }
}