How do i know that my baby get rash from diaper? How to handle when it comes.

My son had a bad case of diaper rash when he had diarrhoea when he was 2 months old. What I did was to wash him bum with regular water + cotton balls, air dry his bum for as long as I dare before putting on his diaper rash cream (I use Mustela). It cleared up in less than a week. After that incident, it's been a protocol to airdry his bum before putting on the diaper rash cream. Better to prevent than cure it