How do i tell the difference on whether i have a cold or allergies? and how to get rid of nasal congestion ?

Fever: The main difference between allergic congestion and a virus is that allergies don't cause fever. If you don't have a fever, look at the duration - congestion from a virus typically lasts less than a week. Nasal steroid sprays work best for the congestion of allergies; if severe, the use of decongestant nasal sprays for 5 days or less will offer the best relief.

Several differences: Common colds are often characterized by fever, whereas allergies never cause fever. Also, allergies are more likely to be predictable based on season, and are more likely to last longer, whereas the common cold is usually "self-limited" and goes away within 10-12 days.