Baby diarrhoea

I'm just a few days post partum and on antibiotics due to an infection. The doctor said the antibiotics were safe for baby, but could cause diarrhoea and if it does I should tell them so they can switch medicine. But, how do I tell if my baby has diarrhoea?! Baby is breastfed and I think his poop is already loose, so how will I know the difference of what's normal and not?!