A litmus test, a barometer: however Manchester City framed and described one night in Madrid during the preamble is irrelevant but that they came through is a major moment in the arc of a new team.
The way that one-goal lead was defended, how Josko Gvardiol emerged as the senior defender to steer them home. Or how they broke with Jeremy Doku. Or how Rayan Cherki intelligently recycled possession to slow proceedings down.