An example method includes detecting over a particular time-period and at a periodic interval, match data indicating whether reference fingerprints representing content transmitted by a non-local content-distribution system on a particular channel matches query fingerprints representing content received by a content-presentation device on the particular channel. The method also includes based on the match data, identifying three time-periods within the particular time-period, the middle of which being a time-period (i) in which there is a threshold consistent lack of matching for a threshold duration and (ii) that corresponds to a detected portion of the received content that serves as a placeholder for local content to be inserted by a local content-distribution system. The method also includes based on the three identified time-periods, detecting, that the local content-distribution system inserted local content into the content received on the particular channel in connection with the detected portion, and responsively performing an action.