How do I tell if I have strep throat or a viral infection or nothing but allergies?

Duration. Sore throat due to step or a virus will only last a few days. Sore throat due to chronic nasal congestion can last much longer. This may be due to allergic rhinitis or possibly a sinus infection. See your doctor.
Rapid strep test... ...And/or culture will diagnose strep - which often coexists with nasal allergies (with blocked nose, mouth-breathing brings strep bacteria straight to the throat). Allergies vs viral - allergies cause more watery nasal discharge.