The way I have done this in the past is a TopologyBuilder (Generate List from Input Edges), on the nodes port test which nodes have only one edge (aka dangle node) based on the number of elements in the edge list (ListElementCounter).
FeatureMerger/FeatureJoiner (x2) the filtered nodes with the lines from the Edge port.
the _node_number of the Node should match either the _from_node or the _to_node of the Edge.
Alternatively the _edgeList{}.fme_arc_id of the Node should match the _edge_id of the Edge (ListBasedFeatureMerger)
The exact way you set it up will depend on whether you want to identify isolated vs dangle, or if the line orientation is important.