Here's a couple of ways to detect Neeko:
* If you have an ability that has an indicator for low health enemies (such as cho'gath ult or Dark Harvest), this indicator will indicate her actual health - if someone's health bar looks low but there is no indicator or vice versa, it's Neeko.
* If you click on a disguised Neeko, it'll still show her actual stats (like her ap, kda, and cs). If you see someone with a different build from Neeko, but click on them and it's her build, it's her. Same thing goes for if one of them is behind/ahead etc.
* If Nocturne ults, a disguised Neeko's true identity is revealed.
* Neeko can't copy passive effects, so watch out for Teemo's not applying onhit damage or Bards without meeps!
* If you ping Neeko's ult in the tab screen, it'll ping the ult of whomever she is disguised as instead. This won't help you ditinguish between Neeko and the person she is disguised as though!
* Neeko's passive is very easy to proc, so if none of the above methods are doable for you, simply hit her with a long-range basic attack or poke spell. During teamfights using area spells is also a good idea to prevent her from proccing the synergy with her passive and ult.