odo. readings can be tricky and hard to tell fo they are off a few thousand or so. If your concerned that it is not correct or that the odo has been replaced then, I look for wear and tear on the bike. Look at the grips, if they are the origional ones they will wear. Also depending on the bike, the seat can tell some story also. If the bike says 5K miles but the grips are work way down and the seat looks worn then it may be off. Also other signs of wear on my bike are the pegs and shifter knob they have rubber on them and it shows that they have had lots of use. Other than rippin the enging apart and riding it, it's hard to tell.
The title should tell the real story also.
As for average miles... Depends where you live how long the riding season is. I live in Maine and I have friends that put on 1K a year, I personally put on about 7500 so far this year, but I ride every day it isn't raining. Most bikes that are maintained well, oil, service, not abused, etc. are fine on the origional engine up over 50K mi, with modern sportbikes.
Most bike will handle 100k miles with proper maintanance, but its easy to kill a bike in 10k if you dont know what an oil change is. My advice would be to look at the shape of the bike, and the state of the wear items, to see what sort of abuse it has been under.
truth of the "12,000 miles is high milage for a bike" is that it is only high because most bikes dont last long. Not because of mechanical failure, but because it is too easy and common to total a $4000 machine at 75mph.
the bike will last, its the wreck that is usually the problem
mainly the reasons why 12K+ miles are considered high for bikes because of the selling price. there are quite a few bikes out there that are in good condition but with lower mileage and could be equally priced or less than that particular 12k bike. so generally, buyers tend to go for the bike with lesser mileage. you're more likely to trade and upgrade to another bike than take it over 100K.