It just depends if baby is in a good position and actually cooperating. My boy was showing off his junk, it was pretty clear what part he had lol.
There's a new (ish) genetic blood test that can tell you gender even earlier than16 but an anatomy US may be difficult. They don't always offer the genetic test--depends on risk factors. It's part of bigger blood screening test.
16 weeks is pretty early. The chances of them being wrong is much more likely then at 20 weeks.