> how do you decide that something is clickbait without clicking it?

It's easy. The headline should tell the whole story:

    "Acme Widget stock down 2.5%"
    "CERN sientists discover Higgs Boson"
    "Smith and Jones tied in polls"

There is no mystery ("Acme Widget stock makes startling change"). While there's not much room for detail, still: 1) I know what the article is about, 2) I know whether I would be interested in the rest, and 3) I can get a summary of the day's happenings just by skimming the headlines. This was the original purpose of headlines and one of the main reasons for the article itself following a structure called the Inverted Pyramid. The headline told the whole story, just in outline. The first sentence of the article told the whole story again, just with a little more detail. The first paragraph covered the whole story again, in yet more detail. Want more details? Keep reading paragraphs. You can stop at any time. Your only loss is some detail. Compare that to the circuitous, long-winded introductions of many blogs, where the writer finally gets to the point about two thirds through.

Clickbait deliberately withholds information, instead of giving all it can in the room that it has. Clickbait churns up questions, headlines give answers. Clickbait lures you into a story that, in the end, was not interesting to you. Headlines help you know whether the story will be interesting to you or not.