I have had the endo biopsy done (found nothing). They were not looking for NKs, they were looking for signs of inflammation (whatever that is) and pathogens which could be treated with antibiotics.
Otherwise, NKs are generally detected through bloodwork, which is why they are controversial -- you can high NKs one day, and not the next.
For me, the treatment (along with an aggressive protocol in terms of steroids, baby aspirin, etc.) is intralipids. It's not expensive (about $400 before transfer and once again, if you are pregnant, after transfer), and my doc uses it in replacement of Ivig, as it has the same good results.