How can I know if NAT instance is becoming a bottleneck

I'm running my own NAT instance inside of an AWS VPC. I want to make sure that the NAT instance will not be a bottleneck and to that end would like to set my own expectation for when to scale to a secondary NAT (if ever). I understand that instance type (currently an m1.medium if that's important) is an important aspect of this but would like to know how to check that the NAT instance is starting