Currently, there is no readily accessible "Has Input" filter for Dataflows in the DataCenter.
However, it can be created by viewing the "Overview" of a Dataset, and clicking on the "Dataflows Created" link — not the "Tidy Up This Dataset" link, though.
Doing so creates a filter on dataflows shown in the Datacenter that is "Has Input:" with the name of the DataSet you first accessed.
This filter isn't available by any other means.
PLEASE enable this functionality as STANDARD for the DataFlows section of the DataCenter.