Why in the world is it so hard to tell the difference between a bad cold and some sinus infection?

Not much difference: With sinusitis, you generally have more facial or sinus pain than with a common cold. However, there's a huge overlap, since most people with colds have sinus congestion. In general, it's not necessary to know the difference, since the treatment is the same: decongestants, pain relievers, and other symptomatic therapy. Antibiotics are no longer recommended for sinusitis unless it persists.

Symptoms similar: Both viral (cold) and bacterial (sinus) infections have nasal drainage both out the front and the back (post-nasal drainage), congestion and fever. Differentiating symptoms include muscle body aches for colds and facial/teeth pain for sinus infections. Because colds should resolve within 5 to 7 days, if the symptoms are controllable with otc meds, antibiotics are with held unless fever is high.