A recent FB answer to why the first kind of tag clauses take a comma and why there cannot be a comma before the word "right" by itself.
"Didn't you' and "has he" echo the words of the question. They are a shortened form of the question.
...He was there early, wasn't he ?
...She was not the director, was she ?
Those kinds of tag clauses take commas.
There is no justification for a comma before "is that correct" because it has its own subject and verb. And we punctuate a fragment that stands for a complete thought exactly the way we punctuate the complete thought.
...Q Where was he?
...A He was at home. He was sick.
...Q Where was he?
...A At home. He was sick.
...She was alone; is that right?
...She was alone; right?
Happy punctuating!
Margie