"How can you tell the difference between an emotional meltdown due to neurological over-excitabilities and a bratty tantrum the child is using to get what they want?"
I don't believe it's as simple as saying "either the child is subjected to neuro-behavioural storm totally beyond her control, or else she's being calculatingly evil and trying to manipulate me by purposely creating the behaviour." I think it's something in between, with elements of both -- the child is communicating, with inappropriate behaviour, something that she feels deeply. My job is to find out what she's trying to communicate, the root cause of feelings behind the behaviour, and help her de-escalate and find other more acceptable ways to communicate both now and in the future.
I dislike the term brattiness. I think that kids generally do the best they can with the communication tools, maturity and impulse control they've got at their disposal. If they're being "naughty" it's because they've got unmet needs somewhere. That doesn't mean you let them get away with the misbehaviour, but the onus is on you to stretch your empathy and figure out where the misdirected behaviour is coming from.