How can you tell when it's a new allergy (seasonal allergies) or a cold?

DD's had a runny nose/congestion for almost 2 weeks. At first I thought she caught 5th Disease from her friend (the timing was perfect for the early symptoms she was having), but she didn't. One day her nose is runny, the next it's fine, and then the next day it's congested. Almost 2 weeks of this. She coughs every once in a while, but it's a dry cough. No itchy or watery eyes, nothing else.

How do you know?