You'll know when the beans are fully cooked when the beans becomes soft and easy to chew.
Beans that are not fully cooked will be crunchy and hard and it usually takes around 8 to 10 hours in a slow cooker to fully cook beans.
In a pot on the stove top it can also take 6 to 8 hours to fully cook beans but soaking the beans overnight can sometimes help the beans cook faster in the pot on the stove.
I always just use my slow cooker while away from home at work and let the beans cook from the time I leave for work until I get back home 9 hours later.
Then when I get home the beans are ready to eat for supper.
So expect at least 8 to 9 hours to fully cook beans in a slow cooker or on the stove top in a pot.