How to check the default route on private and isolated subnets?

The default route is no different to any other route - there's no special marker for it. Look for a route that has a DestinationCidrBlock of 0.0.0.0/0 - that's the default route.

You can use that boto3 command. You can further use this filter to only grep default route

route.destination-cidr-block

Accept answer if helped.