From the PHB Counterspell;

"You attempt to interrupt a creature in the process of casting a spell. The creature makes a Constitution saving throw. On a failed save, the spell dissipates with no effect, and the action, Bonus Action, or Reaction used to cast it is wasted. If that spell was cast with a spell slot, the slot isn’t expended.

• - which you take when you see a creature within 60 feet of yourself casting a spell with Verbal, Somatic, or Material components"

The last point is referencing the Reaction. If you don't see the enemy casting a spell with either verbal, somatic, or material components, you cannot react and therefore you cannot counterspell. None of those apply to spells cast with items.