//判断线程是否释放
//返回值：0-已释放；1-正在运行；2-已终止但未释放；
//3-未建立或不存在

function CheckThreadFreed(aThread: TThread): Byte;
var
   i: DWord;
   IsQuit: Boolean;
begin
   if Assigned(aThread) then
   begin
      IsQuit := GetExitCodeThread(aThread.Handle, i);
      if IsQuit then //If the function succeeds, the return value is nonzero.
                                 //If the function fails, the return value is zero.
      begin
         if i = STILL_ACTIVE then //If the specified thread has not terminated,
                                 //the termination status returned is STILL_ACTIVE.
            Result := 1
         else
            Result := 2; //aThread未Free，因为Tthread.Destroy中有执行语句
      end
      else
         Result := 0; //可以用GetLastError取得错误代码
   end
   else
      Result := 3;
end;

http://www.cnblogs.com/azhqiang/p/3955490.html