But a u/s would show the CL cyst, which often looks the same as a lead follicle. But, a large follicle/cyst in conjunction with a p4 test would tell for sure if you ovulated.
They are right about BBT. It's useful to chart temps, but several months of conclusive temping is necessary to use them effectively. Ovulation can only be confirmed after several days of elevated temps, and by that time it's too late. One elevated temp doesn't necessarily mean you ovulated.