On my first play-through, at level 24, just got to byzantium.

 I have got Engineering up to 80 for the 10% Rare mod extraction. Have broken down around 100 items, but can see no mods different from what were being broken down at Engineering 60. 

Is there some way to identify these 'rare' mods? I see no specific marks on their cards. 

I did search the net; they talk only about normal mods or Nexus. Gamefaq has nothing.
then this forum; but there was only one other thread about Mods, and had nothing to with Rare ones. 

rare one are more expensive

corrosive mod for guns are rare and much more expensive than shock and plasma mod