Value a property carries for a vertex or an edge of a graph
PropertyValue[{obj, item}, name]
PropertyValue[{obj, itemspec}, name]
PropertyValue[…, {name1, name2, …}]
Cases[VertexList[g], v_ /; MemberQ[PropertyList[{g, v}], VertexLabels] :> v -> PropertyValue[{g, v}, VertexLabels]]
→ {}