The red flag feature suddenly failed on our camera a couple of years ago butr didn't affect the operation of the camera.
The way to check is to look into the lens whilst depressing the shutter button. With the aperture ring set to 'A', try depressing the shutter release slowly with the camera pointing at varying levels of light. The more light there is the less the diaphragm should open. In low light it should open fully. You will get confirmation when you have your film developed but if this is happening it should be OK.