The only sure way you can tell in borderlands if a gun is unmodded is if you find it yourself. As for a way to tell if a gun can be legitimately found, you can try recreating the gun using something like Gearcalc. If it's reproducible, then there's a chance it wasn't modded.
However, keep in mind that even legitimately findable guns can be modded by editing the file and giving them the best possible mods. If a gun has perfect mods for it, either the finder got super lucky and won the lottery, or it was most likely constructed.
