This search question showed up this week. For some, just feeling better and having no more pain and symptoms will signal that a sinus infection is gone. For others, and this has happened to me, a sinus culture is required to make sure there are no more detectable bacteria left. I have finished an antibiotic thinking I might be better, days later feel horrible again, only to find that the infection we were treating was gone but a new one had emerged in its wake. It is possible that the first culture only showed one bacteria because it was the most prevalent and once it was gone the other became detected. The antibiotic may have only worked on the first bacteria and allowed the other one to continue growing. My advice is that if you are not sure if the infection is gone to have a sinus culture done to detect any bacteria and to have a sensitivity test done to see what antibiotic will kill off the bacteria present.
I hate antibiotics but with the horrible ordeal my sinuses have been through between strange bacteria that my ENT has never heard of before and the fungal infection, I have had my share of them. We are as cautious as we can be when deciding treatment. Sometimes a good cleaning of my sinus passages and copious amounts of saline irrigating can stop the infection from becoming so rampant that I have no choice but to take an antibiotic. The trick for me is to catch it as early as possible. I also have to remember that if an antibiotic is taken, I need a probiotic.