The phrase "ascertained which" is correct and usable in written English.
It can be used when you want to indicate that you have determined or discovered a specific option or detail among several possibilities.
Example: "After reviewing the data, we ascertained which method yielded the best results for our experiment."
Alternatives: "determined which" or "identified which".
As the different entrance qualifications are accompanied by differing curricula in school and, at the same time, the different entrance qualifications have relatively strong effects on the levels of the prior competencies, it needs to be ascertained which possibilities exist to create conditions in the respective contexts that are more conducive to the development of the required competencies.
In the said study, it could not be ascertained which vitamin E isomer contributed to the enhanced immune response observed.
Data collected using a patient diary filled in by the parents cannot be independently ascertained, which may compromise their accuracy.
In 17% of cases, hepatic cirrhosis was ascertained, which suggests that it also may be another predisposing factor to infection.
If the intervention is successful, it cannot be ascertained which of the 3 therapies involved determined the success.
The complexity of each procedure was not ascertained which may have had some bearing on outcome and duration of surgery as well.
A very friendly chap was trying to ascertain which covers we had purchased.
The trick is ascertaining which of these millions of variations divide us from them.
Griffith-Jones concluded that nobody should be prosecuted, as it was impossible to ascertain which guards had inflicted which blows.
But when dealing with an individual it is important to ascertain which term they prefer, and to stick with it.
In the end, though, it was difficult to ascertain which players might have elevated their stock.
