How can I tell if my 11 month old has a sinus infection?

See pediatrician. Unusual for an 11 month old to have sinusitis. Unclear why you are concerned, but the best thing to do is to have the baby examined by a pediatrician or family doctor.
Uncommon. It is really uncommon for an 11 month old infant to have a sinus infections. What kinds of symptoms is your child having?