If you mean finding out if a lossless file was converted from a lossy format, check the spectrum (FFT) of the audio, if there is a sharp high frequency cut-off (e.g. typically at 16 kHz for 128 kbps MP3), or bands of treble are missing at times, then it is probably lossy.
Edited by stv014 - 12/1/12 at 4:10am