Okay so this is my take. It may come across as divisive, but that's not my intention. This is just my observation after years of reading comics.
If they have similar art styles then there's no technical difference.
But if the art styles are widely different, then I'd start separating. Also it comes down to writing style as well. Each type of comic has a lot of nuances in how character mannerisms and idealogies are portrayed, and this varies by country. (I'm excluding manwha here, because the question was specifically about Manga vs Western (and I can assume specifically American comics)
No one ever accused Teenage Mutant Ninja Turtles and Usagi Yojimbo of being manga just because they had Japanese elements. The art style was too different, as well as the writing style too.
(For those that don't know, TMNT started out as a black and white comic series in the early 80s)