Is it permissible to postpone verifying ritual purity until the Dhuhr prayer if it was after the Fajr prayer?

A woman must ascertain purity (tahara) when its occurrence is probable at prayer times. As long as you checked at the time of Fajr prayer and saw that menstruation persisted, there is no blame on you for not checking before that, unless you strongly suspected purity had occurred. In that case, it would be a shortcoming. You are not required to make up the Fajr prayer because the default assumption is the continuation of menstruation, and you did not ascertain the occurrence of purity at its time. An event is estimated by the closest time.