How can you tell if shotgun is modified?

Answer 1

The question can have two meanings...if you are asking if the shotgun has a modified choke, usually it will say on the barrel if the choke is fixed (full, "imp cyl" for improved cylinder, "mod" for modified). If your barrel has interchangeable chokes, you just need to unscrew the choke tube and read what it says.

If you are asking if it was modified from the factory configuration, modifications that aren't blatantly obvious (sawed off barrel or stock, magazine tube extension, shell holders, combat sights, aftermarket stocks, etc.), may be hard to tell, like a trigger job. For gross changes, have a factory picture and compare. Subtle modifications may be hard to tell.