Let me clarify that what I meant is that the screen on a 140 may appear glassier or cloudier (when off) than a 170. If you physically have the machine, it should be pretty straightforward to tell when they're on. If you physically have two machines that for whatever reason are both missing their badges, it'd probably be more reasonable to open them up and make a determination based on the components. That's what I'd do if it were, for instance, a Kanga vs. 3400c.

But to extrapolate from Elfen's hypothesis (I don't know Duo's really), one could also just swap displays on a bunch of models (I assume between 140s and 170s, but between 5300s and 5300c's/cs's or 3400c's and Kangas (apparently) - and then at least with the 5300 series I don't think there would be a way to tell them apart - with the exception of the 5300ce, which uses a slightly faster processor). - Maybe serial number.  I know that the old Serial Lookup algorithms can determine the model based on the serial; I don't know if (i.e.) 140/170 5300/c/cs 3400c/3500 have uniquely identifying serials.

(Note the reflectiveness of the 5300cs's screen (on the left): )