It is sometimes hard to tell because a lot of the symptoms may overlap. Here is a great chart from webMD that summarizes the difference:
Days to months -- as long as you are exposed to the allergen
Time of Year
Most often in the winter, but possible at any time
Any time of the year -- although the appearance of some allergens are seasonal
Onset of symptoms
Symptoms take a few days to appear after infection with the virus.
Symptoms can begin immediately after exposure to the allergen
Itchy, watery eyes
Runny or stuffy nose
Often; usually yellow mucus
Often; usually clear mucus
Last year, I did a Q&A with my cousin, Dr Joann Lin, who is an allergist in Dallas, TX. If you have further questions you want to ask, you can email them to me at firstname.lastname@example.org and I'll compile another Q&A post with her.