I think probably the simplest approach would be to compare the temperatures of the conductor sheathing closest to the connection and then farther away where it tapers off. The reason being is because I would know the conductor is under the same load along its entire length and the emissivity is most likely identical(Assuming heat hasn’t changed the surface of the sheathing near the connection). Even though the connection on the adjacent conductor is similar in materials, it is possible I might not be able to verify the load.
My answer is based on how you might approach it if you were definitely going to provide a Delta T. I would agree though, that a connection which tapers off is an automatic concern, so a Delta T isn’t really necessary. And the reason I agree with that is because Chuck and David taught me that about a year ago. 