If u have workilist item's folio or something with you, then u can use the below code to retrieve worklist item object and then can access SN. 

SourceCode.Workflow.Management.WorkflowManagementServer wfmServer = new SourceCode.Workflow.Management.WorkflowManagementServer();

            try

            {

                wfmServer.CreateConnection();

                wfmServer.Connection.Open("Integrated=True;IsPrimaryLogin=True;Authenticate=True;EncryptedPassword=False;Host=localhost;Port=5555");

                SourceCode.Workflow.Management.Criteria.WorklistCriteriaFilter oFilter = new SourceCode.Workflow.Management.Criteria.WorklistCriteriaFilter();

                //oFilter.AddRegularFilter(SourceCode.Workflow.Management.WorklistFields.Destination, SourceCode.Workflow.Management.Criteria.Comparison.Equals, "K2:DenallixAnthony");

                oFilter.AddRegularFilter(SourceCode.Workflow.Management.WorklistFields.Folio, SourceCode.Workflow.Management.Criteria.Comparison.Equals, "Test_Folio");

                SourceCode.Workflow.Management.WorklistItems oItems = wfmServer.GetWorklistItems(oFilter);

                foreach (SourceCode.Workflow.Management.WorklistItem oItem in oItems)

                {

                    int actinstdestid = oItem.ActInstDestID;

                    //if (oItem.ProcInstID == iProcInstID &&

                    //    oItem.ActInstDestID == iActInstID)

                    //    wfmServer.RedirectWorklistItem("UserA", "UserB",

                    //        oItem.ProcInstID, oItem.ActInstDestID,

                    //        oItem.ID);

                }

            }

            finally

            {

                if (wfmServer.Connection != null && wfmServer.Connection.IsConnected)

                    wfmServer.Connection.Close();

            }