The best places to start is with a safe word. In BDSM there are a few models used for this.

There is the Traffic light system, which operates on the green (all is good), orange (you need to slow down/change play) or red (absolute stop and aftercare).

There is the basic Safeword which means, when the word is set, everything needs to stop and aftercare must take place right away. its important to pick a word which will not naturally be said or used a lot. Feel free to let your creativity take you away with this. The more uncommon or out there, the better.

In addition I personally use The Non-Verbal signal as well. This is a agreed hand signal my sub has been trained on, so if we are ever in a situation where my sub needs to use a safe word either privately in a public play or my sub is restricted with using words, there are other ways to communicate with me.

As per the above, there are also the Object signal which is more used in very specific plays, where objects or even toys are made available for the sub to be used. In this case, you put the toys or object well within reach and make it easy for the sub to reach. if the sub hugs/holds the object this has the same affect as a safe word and play must stop and aftercare have to take place. This particular safe word is good for subs who struggle to be vocal in highly emotional situations