It is commonly known that males form a ring around their neck while the females don't, but that only happens when they are 2-3 years old.
So telling apart an Indian Ringneck's sex at a really young age before they are sexually mature, can be done in 2 ways.
The most accurate way when they are really young is by a DNA test, off a single feather but…. it is often too obvious by its flirting mode at a really young age of a few months old too ! If your bird is tame or comfortable enough around you, you'ld definitely catch your bird doing some flirting moves like the jumping/bowing and crossing their wings like a heart if he's male like in this video.. But if your bird lowers their body nearly as if they are nesting and leans their head backwards & twisting it left & right a few times then this points out a females mating behavior..
I got to realize my bird was male ever since he was 6 months old when I first saw him acting like that :P Now he's nearly 2 & I'm so eager to see his ring showing up :)