I remember when Allison was little and wondering sometimes if she was sick enough to worry about, to take her to the doctor, rush her to the ER, etc. Most of the time I'd ask some experienced parent and they'd say it was no big deal.
But sometimes it is! A fever that's way too high, or won't go away, sudden stomach pain, etc.
Here's a great article from Parents Magazine that covers 12 symptoms you need to act on.