While I cannot find the specific reference to artificial
sweeteners, hummingbirds can tell the difference. They
are able to not only identify different kinds of sugars
(in nature sucrose, glucose and fructose), they are able
to assess concentrations with remarkable accuracy.

Generally speaking, hummingbirds prefer flowers high in
sucrose while other nectivorous birds lean toward glucose
rich flowers. The result is a division of labor on the part
of flower pollenating birds which is related to the composition
of the nectar in flowers.

An article by Van Remsen explaining methodologies for calculating
concentrations with links to articles on concentration and
sugar preferences.
http://www.museum.lsu.edu/~Remsen/HUMNETf/sucrose_article.html

One of many abstracts found on the net which looks at the
differences in sugar preference between hummingbirds and
nectivorous passerines (in this case sunbirds)
http://www.blackwell-synergy.com/links/doi/10.1111/j.0269-8463.2004.00818.x/abs/

--
Mike Patterson
Astoria, OR
celata at pacifier.com