During playback? Whatever it shows is up to the software player I guess. It's the compressed bitrate—bits of encoded data / second—though exactly how it's calculating (averaging too? over what interval?) and displaying the data, I don't know.
Convert it back to WAV or whatever else and compare the bits. Actually, you can probably open up the FLAC and the original in some kind of audio editor and compare there. Audacity is one free option. Zoom in a lot and look at the data sample by sample. Or invert the data and add it back to the original, and see that you get 0. Or just play back all tracks (inverted plus original) and confirm you get no sound, and that you get sound whenever you listen to each separately. Whatever is enough to convince yourself one way or another.
You can try the same thing with lossy compression and see that you don't get 0. Note that you might need to be careful of everything aligning perfectly in time. I think there was a program for this purpose... Audio DiffMaker?