Philosopher Karl Popper famously asked how to tell the two apart. His answer—falsifiability—hasn’t aged well, but the effort lives on.
Jettisoning falsifiability won’t solve our initial problem, however: demarcation is simply inevitable. Scientists have finite time and therefore must select which topics are worth working on and which are not: this implies some kind of demarcation. Indeed, there seems to be a broad consensus about which doctrines count as fringe, although debate remains about gray areas.