The <wtObjName> tag under <WorkTable> provides the name of the worktable being used by the query plan. The <WorkTable> tag is provided under operators where the worktable is created.
select show_cached_plan_in_xml(107219961, 0) go
<text> <![CDATA[ SQL Text: select distinct c1, c2 from t1, t2 where c1 = d1]]> </text> <opTree> ... <MergeJoin> ... <WorkTable> <wtObjName>WorkTable2 </wtObjName> </WorkTable> ... </MergeJoin>