LadyinCement: I think they can't tell, a lot of the time. But if someone's name appears to be Bengali etc. and they have a qualification in that language, I think universities often kinda "disregard" the qualification in question. The "obvious" cases get weeded out in this way.
And it is very unfair in a way, because it could be that young Rana did not grow up speaking Bengali at all (plenty of minority group members do not speak a minority home language) but developed an interest as a teenager, yet she risks having her hard work ignored. I suspect it is easier for white teenagers to pass under the radar. I don't know what the solution is.
I do think that there has been a move for schools to stop putting teenagers in for home-language qualifications, perhaps because of the above. And Polish A-level I think has been scrapped altogether--due to lack of demand. I suspect that Polish parents were becoming worried that if their kids did a Polish qualification, it would make them look like a soft candidate who wanted to bank an easy A, and that is perhaps the reason for the fall in demand.
It is complicated, because minority-language speakers vary enormously in their proficiency---some may be no more than passively bilingual to start with and may not necessarily find it easy to walk into an exam room and get a great score. Yet the stigma remains.
I don't know exactly what the answer is, but if we returned to the UK I would encourage my daughter to do my country's "proficiency level" exams in the language, which are aimed at foreigners learning the language but are far more demanding than GCSE/A-level, and much better suited to native or semi-native speakers.