How to check the focus lens and collimation lens dirty or not 

( Attention to take the laser cutting head to a clean room while checking )

  Take apart the laser cutting head,then put it parallel to the table,like below pic
  If you already take apart these two protective lens, now please wash your hands seal the cover.
  Take apart the nozzle from the cutting head and open the cover on the other
     side.(as below pic)

  Prepare one piece of write paper,light one the paper,then put on the laser cutting head,use your eyes to see through the hole and see if there have any dirty or black point on the paper? (As below pic).If there have dirty or black point on the paper,then the collimation lens should be damaged.

   Change the laser cutting head to another side,use the same action and see if there have dirty or black on the paper.(Like below pic).If there have dirty or black on the paper,then the focus lens should be damaged.