no way to tell. just use simple logic. if youre buying what was suppose to be a very limited item and is now all over the place, its probably a dupe.
Edit:
Although I forgot to mention there are different types of duped items..
Exploit of game mechanics/hacking software: usually retain item name/properties
*noted that most mechanic issues are fixed and I haven't heard of a massive hacking software used since the red shroud incident*
Dog Dupe: immitates the item but does not retain item name/properties
Examples: gold clothing (from cut up guard sashes - no markers mark, no exceptional mark), grey paladin swords, and/or most other stuff that has a unique hue but remains nameless/no properties (signifying it was copied from an event item)