Camster; Dee P Six gave you the best way to do it for sure,other than markings and calibers. If you see a barrel in person,the 1st and 2nd Gens will have ELEVEN threads on the threaded portion,so the 3rds will have TWELVE or THIRTEEN(I teach history,not math and was trying to get the "ratio"),as the shank part that enters the frame is about .710 in length(but that length is not all threads.).I am using a 1st Gen. bbl. I have before me to count.
Obviously,if its marked .38 Special,it WON'T be for a 3rd. I think the only 2nd gen.44/40s were for a special nickled commemorative model.There are many variations!
To be "safe" try and get a return guarentee from the seller,if you,or he is "wrong".
The "clones" I have dealt with,mostly the older Great Westerns and EMF Dakotas,use 1st and 2nd Gen threads.