That'll be the Endoscope. ENT will only cut out the infection if any. What they won't do is give to any diagnosis on the cause of the problem.
I actually wasn't thinking about infection at all.
A closer look at the sinuses, via endoscope, can reveal many abnormalities other than infection.
For instance, if one has nasal polyps as the source of their sinus misery, they could try targeted medication or have them removed.
Additionally, some dangerous diseases, such as vasculitis, commonly affect the sinuses.
My point, to @Booble, is that there's much that can be ruled out by an ENT before resigning oneself to chronic sinus misery.