You may have to call your phone company and get a list of all 800 numbers and their DNIS digits (the DNIS digits will equal your VDN.) Also, if you have DIDs, some VDNs use those.
It's always a good idea to start looking at your trunks, but sometimes the only place to find the information is with the phone company.
I would also recommend pulling reports on the vdns to see how much traffic they have.