Its usually detected during the 20 week scan. i have velamentous cord insertion AND two placentas that the cord is branching off to both placentas unprotected. This can lead to vasa previa if the unprotected cords are attached low near the cervix or run across the cervix. Luckily, my cord attaches to the sac up high at my fundus so they are in no way concerned with vasa previa.
When they do the 20 week scan they should be looking at the placenta, the blood flow, the cord insertion site and the blood from to and from it so make sure nothing is in the wrong place. Also doing a transvaginal if they think you could have it is the sure way to know, but an abdominal ultrasound can also detect. After they found my VCI and two placentas and specifically asked about vasa previa and they told me as long as they are looking at all of the things I listed above, they should have an idea if you have it or not. ive read up a lot on it since it coorelates with VCI, and its pretty scary.
