No printer controls their motors with >600 dpi resolution. Inkjet printers have print heads with many nozzles; the motors do the rough positioning / slide the head over the paper, the nozzles do the hard work. In laser printers a motor only moves the paper along (all rollers are either free-running or synchronized by gears).
So for an inkjet you'd have to look at the nozzle timing, which might be difficult depending on how integrated the drivers are (e.g. if they're a custom chip on a flexprint behind the heads... uhm...). For a laser printer you'd have to look at the laser modulation signal. That should be much easier, bugs have done that before.
Reverse engineering the firmware might be easier... on the other hand, the firmware is probably bolted shut rather well — the printer manufacturers cartridge DRM is in there somewhere.
I was assuming the nozzles has some sort of actuator that approximates to the term "motor" - I think you're either driving a tiny heater, or piezo, or a charge deflector plate in inkjet printing? That presumably is where the jitter would physically manifest; so you'd look at the input signal to those elements?
Maybe "just after the electronics, but before the print heads/motors" is the appropriate place to probe. It might be more work than anyone's prepared to put in (and of questionable utility), but you could emulate the motors and heads, and generate an image of what would have been printed.
So for an inkjet you'd have to look at the nozzle timing, which might be difficult depending on how integrated the drivers are (e.g. if they're a custom chip on a flexprint behind the heads... uhm...). For a laser printer you'd have to look at the laser modulation signal. That should be much easier, bugs have done that before.
Reverse engineering the firmware might be easier... on the other hand, the firmware is probably bolted shut rather well — the printer manufacturers cartridge DRM is in there somewhere.