How do I know which songs are viruses on limewire or just regular songs?

Basically a single song should be 3 to 5 megs, sometimes viruses are smaller or larger than this, but basically you can't know for 100% sure until you download it.

Just get a good anti-virus and run it on all the files you download before double clicking them. This is just a chance you take when downloading music illegally.

As Lowtax says if you are going to download copyrighted material without paying for it then be prepared to be attacked from all quarters.
A lot of these files are infected on purpose by the RIAA so they can see exactly who is downloading from these "services" so they can issues SERIOUS court action.

Best advice, DON'T use Limewire or any other file sharing program, unless you want a knock on the door from the RIAA's goons.