Here's a bit of analysis advice that I found in a thread on another torrent site, perhaps useful for those who really scrutinize the FA/SA?
First, a quick primer. Looking at FAs, most minidisc sources will show a distinct drop at 13KHz then level off; it's technically lossy but everyone accepts it because that's the source material. A typical 128kb CBR MP3 will show a very distinct drop at 15KHz. If the MP3 is encoded with variable bitrate at a higher bitrate (and depending on the encoder used), that drop still starts at 15KHz but can look much more gradual and the pattern typically gets all warbly (unfortunately, crappy mics will do the same thing). A DAT recorded in EP mode (aka long-play mode or 32KHz mode, used to get an entire show on one tape), will show a gradual drop starting at 16KHz and level off around 18KHz; again it's technically lossy since it cuts out those frequencies above 16KHz, but it's acceptable if that's what the source equipment captured. Assuming it's lossless, everything else will still show a gradual downward slide to the right in an FA, but there shouldn't be any discernable drops at 13, 15, or 16KHz.
Second, when looking at spectral views, you do NOT want to see any black areas at the top of the channel. That's a sure sign of a lossy file. However, again, recording factors play a part - that 32KHz DAT will still show some black because it's technically lossy but we all accept it. A spectral view will also be somewhat easier to tell where the drop starts because there may be a distinct line where the red part of the spectral stops and the blue begins, and this can be very clear.