Answer:
Does the author or artist offer it for free. If not, then the file isn't legal. The RIAA isn't going to pursue downloads of songs that are already offered free, because they are already legal to distribute. If you can download the song off of the artist's MySpace page, then you don't really have to worry about it. But then, you might as well download it from the artist's page anyway.
The only thing that LimeWire should really be used for is obtaining music from independent artists that want to get some exposure, and not necessarily make a buck. LimeWire offers tools to publish files with a license available. if the file doesn't have a license icon next to it in the file list, don't download it.