I know many people find out, and many don't. It seems to me, on the one hand, that we have the technology, so why not use it? We are not J-Witnesses C.V.! But on the other hand, should we try to see the future? "those things that are hidden are for Hashem..."
So, what do the Rabbis say? Or opinions in general?

I know of many who refuse to check out the gender when in utero, the reason is, that they are praying for a HEALTHY baby. Whether the baby is a girl or boy, is irrelevant.

One issue is that we have a tradition that the gender of a baby is not decided until something like 40 days into gestation.

I've heard, presumably from a Medrash, that Dina and Binyamin were switched in the womb.