Baby Monitors light up when the baby cries or makes a sound.
Besides that, (even without a baby monitor) It's all nature; nature takes its course.
A deaf Mother is more in tuned with her other senses, that's when their Motherly Instinct is at its best.
A deaf Mother won't just depend on 'electronics' or 'devices' to stay in tuned to her baby, a Mother can sense when her baby is crying and needs her, that's what 'Motherly Instinct' is all about.
I know for a fact when my son was little, my breasts would leak when he would cry. It was my body telling me that my baby was hungry or needed me for comfort.
This lady told me a story once:
She told me about a deaf Mother and Father who had a baby who had all of his senses, she said that baby learned (quick!) that the Parents were deaf, and instead of crying, the baby would move and use body language to communicate with his parents.
It's amazing how God takes care of EVERY detail, he makes a Mother aware of her young, and a baby aware of its Mother to where no 'modern technology' is needed.
A Mother can sense (in an instant!) when her baby needs her ..