Keeping them disconnected from wifi is sufficient.
Or, still trivial (but more expensive): Point a spectrum analyzer at it and see what kinds of RF emissions it actually radiates.
I don't need to open the box to understand what kinds of noises it makes.
I could however see this happening in the future in response to regulation. For example some "think of the children" rule that makes TV makers responsible for blocking mature content.
It may at some point no longer be economical to manufacture a Wi-Fi only modem.
They just need this revenue by tv to be higher than the price of the mobile plan per tv. For a few Gb per month it may be relevant in the not so distant future.
I don't know if TVs have a cell modem, but it isn't unreasonable to say they could.
As a different poster mentioned, the FCC needs to know about this though and so it will be documented.