It really depends on the kink and the person. My kinks are relatively tame (leashing, collaring, long term domination over days). I usually got a sense of boundaries with a gradual progression, telling her what to wear on a date, a hand on her neck, a fistful of hair, etc… pushing the line until we find it.
From conversations with close friends and partners with other kinks, I think it comes down to the comfort level between those involved.
That’s a copout, but I think it’s a cliche for a reason.