May The Source Be With You*
Unless you're paying huge sums for actual hi-fi headphones, as opposed to relatively huge sums to walk round wearing a mediocre fashion statement (sorry Beats users) or Bose, then I really doubt you can tell between FLAC and MP3.
But sat in the comfort of your own home, that's a different matter. As others have said, at least with FLAC the data is all there for a time when you can either afford something good enough to extract it (at the minute start with a Naim Uniti and work upwards) or when something new comes along to use it to the full.
With MP3 though and CD for that matter (the data is there but 99% of CD players can't extract it fast enough and decode it real time), the "missing" bits you may not be able to hear affect the room you're listening in, feed their resonance or echo or whatever into the music you can hear and hence how your ears hear the rest of the music. Another reason to make friends with your hi-fi dealer and borrow or demo speakers in your home.
*You have to be old enough to remember adverts for the Linn Sondek LP12