How would you test this?

How do you think? By broadcasting an open WiFi network and seeing if it connects and requests a DHCP Lease.

If they form a mesh and route traffic via one TV that acts as an exit node you can check the packet TTLs.

Isn't messing with TTLs a pretty obvious thing to do when communicating covertly? And wouldn't that kind of packet inspection only catch cases of TVs literally acting as packet-level routers, but not catch application-layer shenanigans like aggregating statistics in the exit node so that all the data going over your own network appears to be originating from the same device?

It's still implausible, but thoroughly verifying that your TV isn't spying on you isn't trivial.

There's no requirement that the mesh be IP-based or function as a layer-3 router...

Even if it were IP-bsaed, it would be extremely straightforward for one box to send a UDP message to another, and receive and retransmit at the application layer. No TTL nonsense, No multiple IPs showing up communicating upstream.

But if my TV is doing this, then by definition it's not acting as an exit node so how do I test it?

Put a WiFi NIC in monitor mode and see if the TV is connected to another AP based on the 802.11 packets.

Or if this theoretical implementation by a manufacturer is using a custom protocol/frequency (very unlikely for a variety of reasons), you'd begin by probing with an SDR.

Which part of that qualifies as trivial?

I only semi-agree with GP; it's trivial for someone who works in Tech to pull it off, but that's really all you need to blow the lid on the whole operation given manufacturers would likely have implemented the same behavior across all TVs of the same model/product segment.