The Leblanc passive fakeout out play

The only way to tell them apart is for one of them to attack. The clone's auto-attacks deal no damage, and only the real one can use abilities.

Alternatively, tag her with some kind of debuff or DoT just before her passive activates (Ignite works very well) so you can clearly differentiate the two.

I always doubt my first instinct when dealing with a Leblanc!...granted I play squishy supports so she can oneshot me while my team is chasing the clone.

lel

If I remember correctly the original has items, the clone doesn't.