It depends on which type of records you are referencing. Citing any of Ancestry's collections derived from original records or indexes produced by genealogical societies makes it easy for anyone coming across the profile to search for the document in other ways if they wish. For example, this is a source citation for a death record found in Ancestry's "Ontario Deaths 1879-1938 and Deaths Overseas, 1939-1947" collection:
Archives of Ontario; Series: MS935; Reel 44. Accessed on Ancestry.com. Ontario, Canada, Deaths 1879-1938 and Deaths Overseas, 1939-1947 [database on-line]. Provo, UT, USA: Ancestry.com Operations Inc, 2010.
It provides information for the original record so it can be verified that way. Also, some libraries in North America and most of the LDS family history centers have a version of Ancestry called "Ancestry Library" that includes most of the record collections, but not member trees, and it is free to access at those places.
As for citing member trees, there is no way to verify them without membership. Since they are effectively "closed to the public" it really isn't the right type of source for the open spirit of WikiTree.