How do I know the difference between being sick with a cold and having allergies? How do I treat these two things differently?

I have a sore throat and some congestion, and have also been sneezing. I feel tired and physically weak. Normally I have lots of energy and exercise a lot. I doubt I could handle much exercise at all today. A co-worker said he sometimes thinks he is getting a cold but it turns out to be allergies. How do I know?