install foobar2000.
http://www.foobar2000.org/download
you can use portable mode so it won't affect anything on your system.
now download this zip file....
http://www.foobar2000.org/components/view/foo_abx
open foobar. goto file>preferences>components and click on install. browse to where the the above zip file is and then restart when prompted.
now goto file>add and select the 2 files - one flac and one mp3. now highlight both tracks in the playlist. right click>utilities>abx two tracks.
from here it's fairly straight forward. just click A, B, X or Y to start playback. if A is the flac then B must be the mp3 (or vice versa). same goes for X and Y. you can switch between them taking as long as you like and then all you do do is match A to either X or Y and B to the odd one out. easy peasy considering your statement that i quoted above?
edited 07/01/2011. 13 correct guesses from 16 attempts is considered good enough.