We found out the gender. At first, we assumed we didn't want to know (we were surprised with our first child.) Then we found out we were expecting twins. Then we learned they were mono/di, and that meant same-gendered twins (I'd been hoping, for all of one day, for a boy-girl pair until I found out that we were having identical twins.) I felt strongly that this time I wanted to find out gender, after all.
That was at 14 weeks. By 20 weeks or so, when we had the opportunity to find out, I was more "used" to the idea and open to being surprised at birth by the gender. My husband, on the other hand, thought it was important to find out ahead of time. It also seemed prudent to find out so that our four-year-old, who REALLY believed they'd be her baby sisters, could have some advance warning if her plans had gone awry. And I was excited to know, too. (I was willing to wait, which I hadn't felt was a possibility at 14 weeks when we were shocked enough.)
So, we found out. Two boys. Yes, they were right.
I wanted them to be boys so badly that I scared myself a little with doubting the u/s. So every time we went in (basically monitoring for TTTS and watching growth), they would ask if we knew what gender we were having and I'd say that we'd been told they were boys. I would say it in a "The last tech thought they were boys" kind of way that would invite another look, and the other techs always confirmed it. The birth confirmed it, too!
As far as how possible it is to keep the surprise....I'd say it's a definite possibility. We weren't concerned about keeping the surprise (obviously), but if we had been we'd have told each tech that fact right off the bat: "We're not finding out." As it was, the techs all asked us, "Now, do you know what you're having?" EVERY time. I can't say they would not have slipped up, but it doesn't seem likely.
I suppose if someone said your twins DEFINITELY are fraternal, they'd be telling you they know the twins are a boy/girl pair. (There's no other way to know for sure, before birth, that babies with two placentas are definitely fraternal. They could be fraternal or identical.)
And it is standard to refer to Twin A & Twin B, even when you know the gender(s). The techs and perinatologists always referred to baby A and baby B, as did my OB. It's possible that they might tend to slip up as far as pronoun use during the ultrasound, saying "his" or "her" when talking about Twin B. But they seemed pretty conscious and probably well-coached in their policies.