I have mentioned before that the quality of the background blur is an important indicator of a lens’s quality.
The lens I used for this photo (the 35mm f/1.4 lens) has very good bokeh (wonderful creamy background blur):
(1/50 sec, 1600 ISO, f/2.2
A cheap lens lacks the wonderful “creamy” bokeh you see above; its blur is more “mottled”, “chunky”. I can easily tell whether a good lens was used, or not.