If a new sub shows any resistance to the idea of a safeword I will tell them that it is information I, the Dom, need, and refusing to use it would be lying to Me. That usually gets the idea across.

Although I have often been able to tell when a sub is in distress before they safeword, I would never rely on it until I knew them well. In my long term BDSM relationships it's always been an important step when I told the sub she no longer had a safeword: from now on she would have to rely on me knowing when "NOOO!" means "I really need you to stop right now" and when it means "This is unbearable and I'll never forgive you if you stop."