Although very small (usually between 4-5mm), it is possible to see adult bed bugs with the naked eye. Visualise a credit card. Bed bugs have a lateral body allowing them to squeeze into the tightest cracks and crevices.
Inspecting Your Hotel Room for Bed Bugs (.pdf). When staying in a hotel it is a good idea to inspect your room for bed bugs as soon as you arrive. You do not want to unpack or settle in until you have checked to see if there are bed bugs in your room.
Bed bugs are kind of a nightmare, but the important thing is to know how to identify if you have them, because then you'll be able to get rid of them. The Environmental Protection Agency (EPA) has some good tips though that you can use to check your beds at home, beds you stay on when you go out of town, and even seats in public places like
Start with the bed. Thoroughly check the mattress, paying particular attention to edges, seams and air-holes. If possible, turn the mattress on its side to inspect the under-side too. Bed bugs are more likely to be closest to the head of the bed, but make sure you take a look at the foot end of the mattress too.
Bed bugs do not live on humans, and you are not going to get bed bugs from sitting next to someone on the train, for instance. That being said, you may pick up bed bugs in trains or other vehicles, or even in places like movie theaters, where many people gather at once. You are not "catching" bed bugs from the other people, though.