Just click Analyze -> Plot Spectrum... (first 28.3 sec. should be fine) and look at the graph. If you see a sharp drop like the one at 16000 Hz in the image below, you likely have a wave converted from mp3 (since the higher frequencies have been cut off in the encoding process and converting back to wave can't restore them of course). This example is a 128kbps mp3, with higher quality (=bitrate) mp3's this may not be so clear however.