how do they determine egg quality?

Megan🦄 • BFP: 2/23/17 expecting twins November 2017! B/G twins born 10/10/2017
I'm dealing with unexplained infertility and curious about how RE tests for egg quality?
  I ovulate on my own like clockwork. DH is in good working order. Had HSG Test all good. PCT last cycle, all good. Tons of ultrasounds and I'm producing properly sized follicles at the right time in my cycle.
Started clomid 100mg days 4-9. Go in tomorrow (cycle day 12) for an ultrasound/bloodwork possible 2nd PCT. hoping for answers!