How can I tell if my child has allergies or just a cold?  the symptoms are similar.

Fever. Yes, the symptoms are very similar between a virus and an allergy attack, but allergies don't cause a fever. Colds in children typically last less than a week, but a flare of allergies can last much longer than that. Seasonal allergies will typically match with pollen counts; you can watch the pollen counts in your area to see if they correlate with your child's symptoms.
Fever versus itching. If child has fever then not allergy and if child has itching (eyes, nose, throat) it is probably allergy. Think allergy if child has other allergic conditions, like eczema, asthma or if symptoms perist for over 10 days or increase around pets, when pollen count high. Giving an antihistamine is harmless and ineffective with viral infection but will improve allergy symptoms. Ask your pediatrician.