MRCP2-4700

A 56-year-old patient presents to rheumatology with a complaint of bothersome dry eyes and dry mouth for the past year. The patient has a medical history of coeliac disease and adheres to a gluten-free diet. On examination, dry oral mucosa is noted, but there are no other significant findings. Blood tests reveal a positive antinuclear antibody and an elevated ESR. What is the most conclusive method for confirming the probable diagnosis?