Sonogram technician said you can always see scrotum at 31 weeks. is that true?

About half the time: Not if it's a girl.... Seriously though, at 30 weeks, it's difficult to miss any anatomy, especially with the improved resolution of today's machines. However, I do occasionally find fetuses that curl up, back towards me, and both hands & cord obscuring their genital area. So, again, nothing's ever100% or guaranteed in medicine but we have many tricks to make babies "expose" themselves on us.

Depends.: Fetal external genitalia should be visible by 31 weeks (labia, scrota - not testicles per se), depending on fetal position/activity, maternal weight for height, amount of amniotic fluid, presence of fibroids and duration of your ultrasound examination.