Step 1: The test of a good pessary fit is mechanical retention, so think about what happens when the pelvic floor is loaded.
Step 2: A properly sized ring pessary remains seated when the patient coughs, strains or bears down, that is, when intra-abdominal pressure rises. Retention under that stress is the confirmation.
Step 3: The other clues mislead: lack of bleeding says nothing about position, and discomfort points to a poor fit, not a correct one. Retention under raised pressure is the right marker.
\[\boxed{\text{If it is not expelled after increased abdominal pressure}}\]