Sinus pressure and dull pain under nostril in bone. No drainage/congestion. Worried may be dental infection, but no pain in gums. How do I tell?

See DDS for X-rays. Your dentist will do a thorough diagnosis, including any necessary X-rays, to see if the source of your pressure is in the teeth or your sinus.
Expert. Self diagnosis is at best ineffective, at worst dangerous. You need to see an expert, a Dentist, who has more than 10,000 hours remaining and years of experience, to make that determination. Please call now.