I recall having the ultrasound when pregnant but back then it could not rule out mongolism? Can it now?

Screening. Screening tests provide suggestions that an issue is or is not present. They are generally cheaper and less invasive. They have found that some findings on ultrasound suggest a fetus may or may not have DS. Unless the US was done with this in mind, it would not be useful or reported as such. The definitive test for DS is a more costly and invasive chromosome test on fetal cells.
Downs Syndrome. Mongolism is an older term for Down syndrome (trisomy 21). No, ultrasound cannot rule out downs. However, the prenatal "quad screen" is now offered and can indicate the likelihood of the fetus having trisomy 21.