In parkrun, as far as I know, there's never been any checks on entries, rather, entrants' declared sex and age have been accepted at face value. Maybe that'll always have to be the case, just on grounds of resources (e.g. I'm forever getting emails asking me to volunteer to steward in my local kids' one).
In practice, observance of rules like this will be observed more strongly the higher up you go in terms of standards.