Very good question. We have a 24x7 service at our pediatrician that we can call, but it takes a while to get information back. Not great early in the morning when your daughter wakes up with a terrible cold/fever.
Here is a book that really delves into these issues in a very practical manner. Find out what strange toddler symptoms are a serious problem, which are not. Stop worrying and pouring over internet resources that may/may not be right (I've been known to research to death because you can always find some information that is contradictory to all that you have researched so far!).
There is a giveaway at Parenting Our Kids for this book. Consider entering!