How to programmatically check whether a user has responded to a Survey or not?

Usually we use the follow SPQuery to make sure whether a user  has given a answer to our surveys:

Here we suppose you run these code in the administrator's role:

 

;
 SPSecurity.RunWithElevatedPrivileges(delegate()
    {

     using(SPSite site =new SPSite(siteUrl))
     {

          using (SPWeb web=site.RootWeb)
          { 
             SPUser imitationUser = web.EnsureUser(userForCheck);
             using(SPSite siteWithUser = new SPSite(siteUrl, user.UserToken))
             {
                 using ( SPWeb webWithUser = siteWithUser.RootWeb)
                 {
                    SPQuery q = new SPQuery();
                    q.Query=@"<Where>
                                  <Eq>
                               <FieldRef Name=\"Author\"/>
                               <Value Type=\"User\">"+SPUser.Name+@"</Value>"
                                  </Eq>
                              </Where>"
                    SPList list=webWithUser.Lists["your survey's name"];
                    SPListItemCollection items=list.GetItems(query);       
  
                    if(items.Count>0)
                    {
                          result=true;
                          isRespondComplete=items[0]["Completed"].ToString()=="1";
                    }
                 }
             }  
          }
     }
     });