How to test whether a router supports NAT loopback?

I've been unable to find much information on whether my router (Linksys EA4500) supports NAT loopback, so I'd like to perform a test to tell me whether accessing my external ip address from within the network actually goes out to the internet and back or if the router is smart enough to keep the traffic local.

Perhaps severing the connection between the router and modem or modem and wall would work, but I don't don't have physical access to the network at the moment.

Running traceroute external-ip-address from within the network (over an ssh session) only shows one entry in the output (the external ip address). Running it from outside the network shows a bunch of entries, but I'm not entirely sure what all this is telling me.

Is traceroute the correct way to find what I'm looking for or is there a better way to test it?

You have a misunderstanding of NAT loopback. There is no mechanism for successful external ("out to the Internet and back") routing of this traffic. It MUST be done locally in your NAT gateway. If a LAN-side client can make a connection attempt to the public IP address of your gateway, and it successfully gets a response back from the LAN-side server you had set up a port mapping for, then your gateway is doing NAT loopback.