Assuming you can't see through plastic you will need to completely tear down the printer until you find the dirty portion. There are no simple how-to directions, as far as I know.
First step, assuming the banding isn't too bad, would be to run a pile of blanks through the printer with the largest pieces of paper you have to pick up anything on the rollers, assuming there is no hardware failure (toner leaking into the printer through a crack in plastic caused by an overzealous paper jam removal method) it just may clear it up.
The printer can't print to the edge, so the only real possibility is that the toner is coming off of a roller in the machine, as to how it is getting on the roller you would need to actually take it apart to find out.
We have two Xerox Phaser 7400 printers. One of the printers is leaving a dusted toner pattern on the papers edge. This appears on 8.5"x11", 11"x17" & 12"x18" paper prints. (8.5", 17 & 18 paper lengths) The band or pattern is 2 inches wide and starts from the papers edge and gradually fades away. Any idea what would cause this issue?