How can you view the traffic flows over an AWS ENI over a time period?

I have a customer who currently has an ENI for their default route from a private subnet in their VPC, but wants to change the default route to a NAT Gateway. Before I delete the route pointed to the ENI, I'd like to see what traffic flows over the ENI for the last few days and I want to see if the ENI interface is NATting any traffic. How could I see the logging - time stamp, source address, destination address, port number (source nat if possible)? Or a summary of the flows over time would actually be even better than a flat out listing. Thank you for any insight.