Typically you can see some specks of this collected in the pre filter. The specs are about the size of the tip of a well sharpened pencil lead, are dark grey and are soft to the touch.
Unless there is a large deposit in the prefilter, or the car is not driven much, you shouldn't have to worry too much about it. That said, it would always be better to treat it and eliminate it.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in