How to know is baby diarrhoea?

Hi mummies, my lo is almost 5month old now, haven’t start solid food yet, and now is mixed feeding (fm and bm).. how do I know if the baby is diarrhoea? He had poop four time yesterday, IFC teacher said two times are watery yellow, then at night time at home he pooped twice are mucus yellow, not so watery. This morning poop twice also mucus yellow. All poops are not much until diaper full, is it diarrhoea? Or is normal? TIA