i've been told that bells are handy for this. Either have them tied to a string on your wrist to shake or hold one to drop where you would normally safeword.

But, as it's already been mentioned, regular check ups from the Dom is a better idea than any safeword or signal. The idea is not to ever push to the stage where a safeword is necessary, particularly not in public. i would assume that the safest way to play is never to do anything in public that hasn't been done in private, as denu said.