So you're telling me you can only tell if it isn't a bubble if the market steadily declines eventually? Also how much should it steadily decline by before we're sure it's not some local decline phenomenon unrelated to the bubble?

Whatever "bubble" means, it should probably only be used as a description of an abrupt crash in retrospect, otherwise we're always in a non-falsifiable bubble state. Saying "We're in a bubble" really just means colloquially "I think this market segment is overvalued", probably not much more than that.