to visually check is virtually impossible without taking it out. I helped my mate do mine (he's a mechanic with his own garage) & it was a right struggle getting it out.
If the dealers hadn't left mine unplugged I would probably still have my dpf fitted & if it blocked I would have a go at cleaning it 1st before buying a new one. Mine is in the garage & can either go to the next owner or I will save if the MOT's change.
Would rather have left it on to be honest didnt want to give the dealers another £1500 after giving em £1500 for a new ecu & vacuum sensors,