I agree, like many things, it's a continuum rather than a binary, but if we are going to draw a line somewhere, my heuristic would be when the script is named after a foreign language, it isn't native. So, for example, the "Arabic," "Chinese" and "Latin" scripts are native, respectively, to Arabic, Chinese dialects and Romance languages, while in Farsi, Japanese and English they are in customary use, but are not of native origin.