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That's a bit of trial and error. I have a few plugins that set cookies, and usually I try to make those cookie names the same as the extension name. So if the extension name is plg_content_beforeafter, that's what I name the cookie. Sadly, not all developers use straightforward naming conventions like that.
It's a trial and error thing really. Run your site, look at the cookies that get set and try to identify where they came from.