how do you tell?

[Junon]

Thats probably the only way you can tell, or if you seem to get an amazing deal.

EG; You trade a regular level 200 Mag for a Tsumikiri J Sword, its probably gonna be duped.

Trading with a stranger can be a bit dodgy, especially if theyre from the Vega Ship. However, you may find trading with the Japanese to be pretty reliable, since they all pretty much play the game fair and square, but id still be carefulll...

If you stick with a group of legit regular friends and make sure you only trade with them, you should be able to get the items you want