Nobody else does. If it's runny, you need to cook it some more. If it forms a soft ball, it's done. If the glass blows up, you've overdone it. You take the pot off the burner while you do your checking.
Veterans say they can tell when the soft ball stage requirement has been met, without even checking.
When you are satisfied that the mixture is cooked enough, take it off the burner and add the marshmallows and peanut butter. Using a large sturdy spoon, stir the ingredients until the marshmallows and peanut butter are melted and thoroughly mixed in. When and if it hardens, cut it into squares. Stir in peanut butter. Place on non-stick treated cookie sheet.