OPK confusion.

Becka • Student and stay at home wife. Currently TTC baby #1. Starting assisted conception in June.
Can you tell the difference on ultrasound between a follicule about to release for ovulation and one that has? I took clomid CD 3-7 had a positive opk on CD 14 and at my ultrasound on CD 18 they said there were two dominate follicules that looked ready to ovulate but I haven't received any other positive OPKs since CD 14?