When I did a cycle to evaluate how things were working, I did an ultrasound to measure lining and follicles early in cycle, like CD5 or so, then back CD13ish closer to O. They then had me come back about a week later for a progesterone check, which showed a lower result (I think 9 pt something) high enough to confirm O but kind of low overall.
They could also do a US after O to see, my doc did the progesterone draw though. I think they would need to do a US before and after to confirm.