If I am right, ISP redundancy acts by changing the default route, so you can perform a snmpwalk to some OID of the routing table that includes the default route, maybe this one:
snmpwalk -v1 -c community -On localhost .126.96.36.199.188.8.131.52.184.108.40.206.0.0.0
.220.127.116.11.18.104.22.168.22.214.171.124.0.0.0.0.0.0.0.0.x.y.z.t = IpAddress: x.y.z.t
and it should return the ip address of current next hop. If the primary ISP is down, the OID should change to the nexthop of the second ISP.
If you don't want to perform a snmpwalk, you can perform two snmpgets, each one to the OID of the next hop of one of the ISPs. The snmpget that returns a value would be the ISP that is acting as primary, the other should return a "There is no such variable name in this MIB" error.