The exact mechanism of how they determined this, I don't know but it was likely similar to studies they did that I have seen the video from, where they determined that babies only hours old know what sounds are their own mom's native language and what sounds aren't, because they give those sounds more of their attention. Likely the same kind of measurement can tell if they preferentially listen for the sound of their mom vs. The sound of other people.

https://www.washington.edu/news/2013/01/02/while-in-womb-babies-begin-learning-language-from-their-mothers/