How will I know that my baby needs to burp?

I will always burp a baby after every feeding to ensure that he/she expels any swallowed air in their tummy until they're about 3-4 months old. If baby doesn't burp, i will carry them upright against my shoulder for awhile or lay them against my chest if they fall asleep before burping to ensure the milk stays down so they don't vomit.