Determine miles on a bike
250 miles is nothing. 2500 miles is also pretty much nothing -- this is under a year's worth of riding for many people. Many people still ride (and buy) bikes which are ~30 years old, and probably have 25000 miles or more on them (and will ride them for many years to come). 

You can look at the model number (and compare it by year to which model year it is). But beyond that, you can't really tell anything, since a lot of parts which wear (tires, brakes) can be easily be replaced (and you won't know if they've been replaced unless you knew what was originally spec'd; and they could have easily been replaced due to other reasons, such as cutting a tire on some glass). Mileage isn't a good indicator of the condition of the bike anyway (you can easily have a bike which you've racked up 5000+ miles and looks and runs like new). 

What you should look for is no cracks in the frame, a solid headset, true wheels, straight fork, etc. -- a 250 mile ridden bike which has been in a crash which damaged the fork is probably worse off than a 2500 mile ridden bike that was cared for. 

We have several questions such as this one and this one on how to check out a used bike before purchase. 



All that being said, most bikes which have really been ridden only 250 miles will have all their original equipment, and look pretty much new (no wear on the cassette, no worn chainrings, the original chain which likely hasn't picked up much dirt, handlebar grips and saddle looking like new, very few scratches, etc.). But there are bikes parked next to mine right now which have been ridden thousands of miles which look that way too, because someone took care of it.