50-50 accuracy for most chicks. What she is doing is called feather sexing. If the parents are set up properly genetically it works really well. It’s the common way commercial hatcheries use for broiler hybrids and used with some egg-laying hybrids.
If you want details on how sex links work you can read and study the first post in this thread. Just the first post, don’t worry about the rest. Tim does a great job explaining how sex links work and feather sexing is one of the three methods he discusses but you will probably have to read it a few times to catch on. I sure did.
For feather-sexing to work, the mother has to have the dominant slow feathering gene, which she gives to her sons but not her daughters. The father has to have the recessive fast feathering gene and not the slow-feathering version which he gives to both his sons and daughters. So the daughters wind up with only the fast feathering gene and the sons wind up with one of each. Since slow is dominant over fast, they feather out slower.
This only works for a few days after hatch but for someone that knows what to look for, it’s obvious. There are several videos that show how to do that, but most of them eliminate the part about setting the parents up properly genetically first.
Well we have buff orpingtons, so I don't know, but we founf one out of 4 that had only 1 set in the primary wing, the other 3 had double, so we put a bread wrapper band gently on our alledged male. it will be interesting to see if we guessed correct.