Let me try to get things straight, you are asking how it was determined that a couple girls that Alcala had photos of turned out to be Joyce Gaunt and Antionette Wittaker, is that correct?
Besides having to look up articles, etc., it is possible that they could have used NamUs.gov and searched for girls who had features similar to the girls in photo, such as having, blonde hair, fair skin, blue eyes, etc.,
then they could have narrowed the search to the time that they knew Alcala was incarcerated, once they found hits on the possibilities who the girls might have been they could have looked for unidentified remains that were located in areas that Alcala was known to be at, you could narrow it down to include only caucasian females (or whatever race/ethnicity they were), then you could compare the unidentified females to the missing persons and look at the areas where they went missing and where the unidentified remains were found,
also you could use Charley project and DoeNetwork to see if they have more information on the missing person you found on NamUs.gov as well as the unidentified remains,
you could also do a simple search of Google or Bing or whatever search engine you want to use and see if someone may have started a FaceBook page for them and there may be more information about their disappearance there,
sometimes if you are lucky you could find articles that are free,
if all else fails you could ask someone here if they have access to certain databases that have stories about the missing or unidentified person you need a look up for, their are many generous folks here on WebSleuths that would be willing to lend a hand and assist you in whatever way they can.
Hope this helps.