It can be hard to authenticate a DT (double telescoping) saber as original. Reproductions are becoming rampant, and a fake saber can be had for under $20. I am dedicating a page in my figure guide, Vintage Kenner Star Wars action figure variations, to the issue of reproduction sabers. Vs originals. etc..
Here are some pictures of an original saber- marked N- originals can be found with marks for A to ZZ and some with just a dot on the hilt also can be found. They are slightly transparent, and with age they can be come very brittle. 
Reproduction sabers are becoming very precise- but there are some things to look for- Some, but not all Repro’s by Revalvit are marked REV on the hilt, theres are not marked abut have II mark on the hilt. Any saber that is unmarked should be suspect as being a repro.
Note seam, on the shaft, rounder tip – REV on the hilt. Ejection point marks on shaft. 
pix of revalvit sabers- from website.