If I understand your question correctly, you can look at the routing table to look for a route to 0.0.0.0. This would most likely be the live route, but if you have load balancing or static route it depends where the destination is. If this does not help, can you better describe your question better.
I assume this is an edge router with a default route pointing to a hub site? if so, I'd go for the routing table and see which interface has the default route.
that would return the InterfaceIndex which is a column on the Interfaces table that would (with the NodeID) give me the InterfaceID that I can then use to jump to the current active interface.
ahh, so can a single uDNP bring back the routing table in a format to present on the node-details page?
However you don't need the whole routing table, you need one entry: IP-FORWARD-MIB::ipCidrRouteIfIndex.0.0.0.0.0.0.0.0.0.0.0.0.0
which on one of my leaf-routers returns '6'
Then write a little bit of SWSQL to join together interfaces, custompollerassignment, and custompollerstatus to get the interface attributes you want to display.
Writing sql is somewhat of a requirement if you're doing anything advanced with SWO, so download the SDK and play around with with SWSQL tool.
aside: I would not bother with this in my environment: the primary interfaces I monitor with up/down status alerts, and any backup interfaces I allow to be 'unpluggable'.
On the hub routers we make use of interface alerts and use those to work with the telcos to restore the primary service to sites.
I don't really care which interface is active at a time: if the primary interface is broken, or that the node is down (both the primary and backup interfaces are down)