I'm a beginner trying to get on a road bike. I've been shopping around for a new bike for the last week and am also toying with the idea of getting a gently used "pre-owned".
One of the guys I've talked to, has said that he has only put around 250 miles on the bike. Is there a way to determine that this info is right? What do I look for to make sure that he is not bluffing.
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.
Additional points not mentioned in the existing answers:
Cleanliness
It should be shop-clean with little road grime, and shiny chrome. However if its been hanging in a shed for a few years it will have dull chrome and a light coating of dust. The dust of storage sits on "top" compared to dirt from riding which is up underneath. Seller may have washed it, so focus on areas like around brake calipers and around the bottom bracket/chain stays.
Records
If seller bought it new, ask to sight any receipts.
Also ask if seller has used a recording service like Strava or mapmyride or edmodo. If they have then their rides should be captured and totalled. Tell seller you want to see how fast the bike has gone.
Smell
Plastic and rubber items have an odor, which comes from the more volatile organic compounds outgassing over time. A new tyre should smell like a new tyre for a bit, but perhaps not as long as you want.
Trust
Do you trust this person? Do they still ride? Would they be okay letting you go for a couple-hour ride on the new bike, alone or together? Does your bullshit-detector trip or your spidey-sense go all tingly when seller says something?
Frame problems cannot be replaced. Everything else can be cheaply done. The cassettes can be replaced, so can bearings in the head set and bottom bracket. A broken bearing in the bottom bracket can cause major damage to a frame so that no future bearings will work. Turn the pedals with some weight, if there's any clicking, it's cause for worry.
If a bearing shatters and the pieces are then weighted, they will gouge the frame, so that the clicking, regardless of new bearings, will persist.
The fork is also a major piece that cannot be replaced easily, inspect it for warping, cracks, discoloration.
Note your location and the bike's origin; it helps if the local bike shop has replacement parts.
Stuff not to worry about:
