My partner and i always discuss before or during what we woud like/do not like and are very direct with one an other about our sexual fantasies or what makes us uncomfortable. However sometimes if we're too heated and in the moment to actually discuss what we want/dont want we will move one an others hands. Body language is also important and there are certain cues to look for, but like moaning, it's not the best indicator of consent. Safe words are also very important too, specially for those into heavy bdsm or who role play (such as rape fantasies) because you need to know when you're partner has had enough and you've possibly crossed the line, regardless on how well you may know one an other. I didn't choose that as an option for myself as my partner and i only practice very lite bsdm and have previously discussed what is and isn't okay to do.