Im 21 weeks pregnant and i just had an u/s. the u/s tech checked my cervix trans vaginally , would they have see a incompetent cervix if it was there?

Incompetent cervix: An incompetent cervix usually shows up around 35 weeks of pregnancy. You are much to early to have one.

Yes and no: If they checked your cervix they would have seen the length of the cervix. They can also see if your cervix responds to increased pressure on your abdomen. The tech would also look for "funneling" which means membrabes falling into the cervix. Short cervix plus response to pressure plus funneling significantly increases your risk of incompetent cervix.