What you are referring to is "scene negotiation".  Before you ever start playing, you want to discuss with your play partner a number of issues . . .  For example, what is your past "play" experience (if any)?  Do you have any medical conditions or ailments that they should know about?  Do you have any hard "limits"?

The standard practice is kind of a traffic light system.  "Yellow" means slow down.  "Red" means stop.

I've been taught (what I believe) is a better way.  The Top or Dominant will instruct the bottom or submissive to use that, but also periodically ask for a number on the 1 to 10 scale, with 10 being the worst pain you can possibly experience.  So, let's say you start a spanking scene, and the Top commands, "Number?"  If the bottom say, "4", then the Top can go a bit harder.  Another aspect to that pre-negotiation is to use the "Mercy" rule, but be specific, like, "Mercy on the bottom of my feet, please, Sir!"  That way, you can continue playing, but you just stay away from the area that's "had enough".

By the way, if you meet a Top/Dominant that doesn't negotiate a scene beforehand (or a bottom/submissive who has "no limits"), run for the hills.

In my circle, most of the time they use the words "yellow" and "red" with the same meanings as traffic signals (slow down and stop). However, I found that I would bottom out before coming up with red - and I think it's because I wear red all the time so it isn't a stop word for me (except those big ones on the corner). So, I use "enough" when I'm at my stopping point -

Sometimes, when I'm being caned, and am in the endorphin space so much that I can't really talk, I simply take the cane away from the Top.

Hope this helps.