The Muhaddithoon (Specialists in Hadeeth) have already done the grading. When an `Aalim wants to find out the grading of a particular Hadeeth, he will go to the Kitaabs of Jarh wat-Ta`deel and see what the Muhaddithoon have said about that Hadeeth and how they have graded it, whether it is Saheeh, or Hasan, or Marfoo`, or Mawqoof, or Muttasil, or Mu`dhal, or `Azeez, or Mash-hoor, or Mu`an`an, or Mub-ham, or `Alaa, or Nazalaa, or Mudallas, or Maqtoo`, or Dha`eef, or Mawdhoo`, or Musalsal, or Mursal, or Ghareeb, or Munqati`, or Shaadh, or Maqloob, or Mu`allal, or Mudabbaj, or Munkar, or Matrook, etc.
All of the Ahaadeeth have already been graded; there is no grading of Hadeeth any longer. The grading has already been done hundreds of years ago. Now, the `Ulamaa just check up the Kitaabs of Jarh wat-Ta`deel and the Shuroohaat of the Hadeeth Kutub in order to find out how the Muhaddithoon of the past had graded it.
والله تعالى أعلم