MP3 is the ideal audio format if you like your music files to be compact. If you're more interested in sound quality, though, FLAC makes a far better choice. It's supported by many music players, and it uses lossless compression, so you can be sure the file is a completely accurate reproduction of the original track.
Except, well, that's not necessarily true. If someone creates a set of FLAC files from lossy MP3's then you'll have the same reduced sound quality, only with a much larger file size - the worst of all worlds. So where did your FLACs come from? Audiochecker can help you find out.
At first glance the program's interface looks something of a mess, but don't worry, there's not too much to learn. Click the "File(s)" button first, and choose one or more FLAC files to check (the program also supports APE, SHN, WAV and LPAC files). Then click Start, and wait a while, as Audiochecker uses a technique called auCDtect to analyse whatever you'd specified.
When it's done, you'll see a report on each audio file source, and the degree of certainty the program has in its conclusions. If it says something like "CDDA with probability 100%" then that Audiochecker is sure the recording originated with an audio CD, which should make for great audio quality. If it says the source is "MPEG" then it's probably been derived from an MP3 file, and isn't a lossless recording at all.