After playing some long tones with my tuner today I decided to google "how many cents is considered out of tune" and came across this fun little test.
Listen to four pairs of tones, and say which is higher. http://www.noiseaddicts.com/2010/03/can-you-hear-like-an-audio-engineer/
The difference goes from 50 cents to 12 cents and the author says he finds 12 cents difficult.
I got all four correct, but the last one I had to listen to each tone about 3 times before I could be sure.