I had an u/s on 3/20, and the dr mentioned that there was a ruptured cyst indicating ovulation had prob occurred. is there any way to determine when that may have happened (1 day prior? 5?)

No: There is not a way to determine when that occurred. I am wondering why you would need to know that. Perhaps you are concerned about pregnancy risk if you had sex around the time of ovulation? If intercourse was within 3 days use plan b. If within 5 days use Ella (ulipristal acetate) if you are trying to avoid pregnancy.

Not really: Ovulation is determined by ur biology. When you are consistent with your cycles and it occurs mid cycle or two weeks before menstratution.