Both, in a way. In order to be sure it's the skill reducing token usage, you measure it with and without the skill, and then you do that periodically to account

Both, in a way. In order to be sure it's the skill reducing token usage, you measure it with and without the skill, and then you do that periodically to account for all the other variables. Alternatively, you pick a belief based on prior evidence from either approach you mentioned, which is the natural thing that many of us do.