It's almost certainly still at its original potency. Check out this link: http://www.hsibaltimore.com/ealerts/ea200408/ea20040826.html
Whether you should take it for a sinus infection is a better question. Sinusitis is second behind otitis media as the most over-diagnosed condition here in the States. Most people who say or even have been told by their doctor that they have sinusitis, in fact don't. I know this because of the huge number of people who get referred to me with this diagnosis, who upon CT scanning, don't have sinusitis. I can say it with even more conviction with your case since I'm guessing that you have diagnosed yourself and are using some unfinished antibiotics. Right? Please don't take offense since everyone does it.
Jeff
Facta non Verba
You should call a pharmacist and ask.