I am using Google Cloud Platform and have manually setup a NAT gateway to peer two networks A and B so that workloads in network A have a default gateway which routes packets into network B. The gateway is implemented as a managed instance group (MIG) where each instance has a primary interface in network A and a secondary interface in network B. An internal load balancer in network A is then configured as the default gateway to route packets to the MIG (as described here). The MIG instances themselves are using IPTables to route packets.
My question is how can I health check each instance of this MIG? There is no user-space application in the hot path, just the kernel routing packets. Testing I can route through any MIG instance to some endpoint in network B risks failing all MIG instances if the endpoint goes down. I could test making a TCP connection to each instance but it wouldn't really test the whole routing logic.