There is an easy way to test that. Foobar (and probably other programs) allows you to conduct a double blind test with two files of your choosing. Pick a song that you like and knows really well and don't mind listening (parts of it), many, many times, and see if you can reliably tell them apart using your best setup. Do take rests if you want to go for a long streak as there is a fair chance that even if you can hear the difference, it will be so subtle that you will have to really, really concentrate on it.
To tell the truth, I a modern PC can transcode a huge collection of FLAC to a lossy format super fast nowadays (I also like to see all my core/thread get a good workout) so while a minute space saving is negligeable, so is the cost of doing a transcode to a more efficient format. As I said earlier, I am more concerned with the impact on battery life those more space efficient format might have. There are more efficient than FLAC when it comes to lossless, but last time I checked, they are far more demanding when it comes to decoding, so I wonder about those more efficient lossy format too (while not always the case, I tend to associate better compression with more expensive decoding).