If the seller has had the car serviced by a reg BMW dealer he should have the cars service book stamped. Ask to see that book if he says it has a service history. Ring that dealer and confirm those services.
Obviously it doesnt take into account any other services/repairs that have been carried out by other garages.
Ive heard tell you can log on to a BMW website, key in the reg and it will return the service history. Only rumour I think.
Really, he could have crashed the car and had it repaired since the last service, so if you want to buy the car you're taking the seller at his word unless you get it checked out yourself by your own mechanic to assess the cars condition.