The only sure way you can tell in borderlands on 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 its 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.

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.