I'm guessing sometimes frequency cutoff is due to using less than ideal microphones, although not the extreme cutoffs (I'm fairly sure it would be gradual in that case). It also seems there is often a fair amount of noise around 20kHz that I'm guessing is from recording equipment, sometimes on tracks that otherwise look suspiciously like they are from lossy audio. Additionally, in some cases there could potentially be high frequency cutoffs in processing while otherwise being lossless, particularly stuff like electronic music where there can be a lot of processing in general. So I'm not sure if it is necessarily all that easy to tell in general, although there are certainly some from Bandcamp that seem highly likely to be conversions from lossy formats :(.