Pay close attention to the quality of the hinges that are being used, because cheap hinges tend to make the bike flex; in some cases, they will cause the bike to break. Also, the steering post needs to be well built and also rigid.
Check the wheels and the brakes to make sure they are of high quality. See if they are made by a manufacturer who is well-known for them, like Shimano (see Resources below).
Make sure that the frame of the bike is rigid. Expensive bikes are made of exotic alloys that are considerably lighter than those made of steel. On the other hand, inexpensive bikes are made of steel, so their manufacturers had to look at other ways to lighten them. Make sure they haven't compromised the construction of the bike by adding inferior parts.
Make sure that bike is big enough for you. Many manufacturers will use parts that are smaller than normal as a way to limit the weight of the bike. Consequently, if you are a person of average height or larger, you need to be sure that you are buying the right size bike. For one, see if the seat can be raised so that your leg is almost straight at the bottom of the pedal. Also, check to see if the handlebars can be raised so you can ride comfortably.
Think about your typical riding conditions. If there are a lot of hills on your trip, you should have enough gears on your bike to make the trip comfortably. Most folding bikes have some gears, but only you can determine if the bike you buy has enough gears for you.
