quite often doctors aren't quite as interested in making diagnoses as they are in assessing the pros and cons of a variety of treatments. oftentimes a doc doesn't have a precise diagnosis but the doc doesn't need one in order to assess that antibiotics are likely to help with minimal side-effects. so they prescribe those... and if the patient comes back then they will have to have a bit more of a think. (of course patients quite like diagnoses so they often tell us what is most probable)

> Treatment of a cold may include rest, pain relievers and over-the-counter cold remedies, such as decongestants. Treatment of seasonal allergies may include over-the-counter or prescription antihistamines, nasal steroid sprays and decongestants.

the main treatment difference is antihistamine. you could take antihistamines... then after a week or so of being better... you could stop taking the antihistamine. if you stay better then that doesn't tell you for sure that it isn't an allergy (you might not be exposed to the allergin anymore) but if you get worse then you will know it is some kind of allergy...