Take one of your FLAC files and convert it to 320k. Then highlight both files, right-click -> utilities -> ABX two tracks.
You need to do it at least ten times I'd say as to rule out luck.
Given a single pair of tracks and ten ABX comparisons, one would need to correctly identify X eight times in order to reject the null hypothesis (the null hypothesis being that there is no discernible audible difference between FLAC and 320 kbs for a particular listener on a particular equipment setup listening to a particular song) at a 95% confidence level.