i am looking at a silver conn double french horn made in 1978, but i can't tell what model it is. Is there a way of determining if it is an 8d from the serial number?

See
- the serial number does not include the model. If you search on ebay or hornsaplenty.com you will find pictures of 8Ds which should provide the confirmation you desire.

<Added>

Sorry http://www.hornplayer.net/archive/a34.html got lost when I posted a link