Unless you actually have access to the seats sold and the percentage chance of the pax turning up allocated to each one (which is pretty internal BA stuff) you cannot really know if a flight is "oversold".

Seeing no seat availability is something quite different. BA have the best understanding of how the bookings go, including how many will be accepted on the day, and from where. Also BA do not practice the gross overbooking that US (and third world) carriers do. They can be let down when the chance calculations are not accurate, but that's a different matter.