Sinusitis or something worse?

A 61-year-old man with a history of seizures presents for a routine visit. His last seizure was several months ago and occurred the day after falling from a ladder. He does not recall hitting his head during this fall. Otherwise, his seizures have been well controlled. His only complaint is a persistent runny nose. Computed tomography scans reveal opacity with a fluid level in the left sphenoid sinus. The neurologist inquires whether a test exists for the presence of cerebrospinal fluid in the nasal drainage.