Normally the same plane is used consistently for a particular flight number, but you cannot tell from the flight number alone (sddsddean's post covers that). Sometimes there is a last minute equipement change to a different type of plane. It happened to me recently, and they were re-assigning seat numbers at the gate. The guy in front was insisting he wanted the same seat number, and was told it was not possible. He was geting angrier, and wouldn't believe that seat number was not on this flight. Silly prat ended up being denied boarding.