If a follicle has ovulated before ER/IUI, can ultrasound detect it?

Since my BBT was elevated on the day of IUI (converted from IVF), I asked the nurse whether we can do an ultrasound to find out whether I have ovulated early. They said no - Ultrasound can not tell/give any additional information. In addition, they never use BBT anyway. So I went ahead with the IUI w/o an ultrasound. But I am still wondering whether the doctors can find out an early ovulation before ER/IUI via ultrasound or other technologies.

Any of you know more about this? Many thanks!