how to know if baby has infection?

I was running errands with my 5 month old baby girl and she fell asleep half way through. When we got home I let her finish her nap in her car seat. When she woke up I changed her diaper and discovered she had pooped sometime earlier and it went all up the front of her in her folds. She could have been sitting in poop for hours! I feel horrible.
 Is it inevitable she will get a rash or infection?
How will I know if she has an infection? Google isn't helping me much ☹️