Lance has is right. Telling original from dupes can be tough if the person doing the work was good. But each generation looses a bit of clarity and usually contrast too.
I learned a lot from my friend who collects 16mm movies.
There were a lot of different film stocks, Kodak made many different types as did many other companies.
Fortunately Kodak used datecodes on most of their film. And Fuji used datecodes too. This list mentions Kodak movie film specifically, but I've seen the codes on other types of negatives.
If the film has turned color- Redfor positives and some negatives for B+W done on color stock-I think green? for negatives ) it's usually from between the early 50's and 1981-2. Not all film with good color is older or newer. Kodachrome if it fades does it so little it won't be an issue in our lifetimes. And The kodacolor replacements labeled SP or LPP for movie film are much better-SP turns slighly brownish while LPP fades very little (Like only a very small bit of color density after 20+ years) I'm not sure which still filmstocks those match since the same emulsion might be used under different names.
http://www.film-center.com/dates.html
The plastic of the film changed a few times
Nitrate----*Can be a bit scary since it can self ignite and is hard to extinguish because it's also self oxidizing. I keep the little bit I own on the fridge. This was pretty much on its way out by the early 20's
CelluloseAcetate. Older safety film
Cellulosediacetate - A bit newer
Cellulose triacetate - Up to current.
All those are prone to breaking down and giving off acetic acid. They won't do it for sure, but once they start it's time to have a good dupe made as they'll eventually shrink and become brittle. I don't recall the approximate dates of when they changed. It's
really tough to tell them apart. I've never found a reliable way short of scientific tests I don't have access to. They may burn, but if so only like a small candle.
Mylar/Estar ---Modernish to now. Not sure exactly when it began, maybe 70's. A bit thinner, It doesn't burn, doesn't melt until something like 800F and is hard to tear or break.
A much more detailed look here, again primarily for movie film, but a bit of it crosses over. Also some detailed info on non-US datecoding and production which can be very different. (Technicolor- dye printing onto B+W- ended in the US in 75 with the last feature film being Godfather II. Italy ran until 1980 and in the UK till 78 when it was sold to china ad used till the early 90's. It's been brought back for a few special projects)
http://www.brianpritchard.com/Date%20Codes.htm
Steve B