How can I tell if I a weapon has been modded?

I was recently playing Borderlands online and was given a really great Pearlescent Serpens Assault Rifle by someone who was much higher level (at the time). I'd not seen any Pearlescent guns before so was rather excited - I snatched it up and began systematically annihilating all of Pandora's wildlife.

It's a great gun, very powerful and quite accurate, which made me wonder if I'm unknowingly using a modded weapon. I looked up the Serpens rifles online and found that mine seems to be of above average awesomeness. In particular, it fires 3 rounds at once.

In any case, whether this particular weapon is modded or not, is there a way to tell which weapons are modded and which are not?

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.