Hi again!
I will caveat my response firstly be saying that I can only give a general response, your professors/tutors may want different things from you so it is worth thinking about what has been specifically asked of you.
That being said, you have found one of the biggest bug bearers of a Law Degree right there! You likely won't know what topics you may be faced with in an exam, even more relevant for closed-book exams. Therefore, to some extent you can only do what you can and hope for the best.
At university level, your best friend ought to be journal databases - i.e. Lexis, Westlaw, Jstor, Google Scholar, etc. These are the key sources of critique and commentary of the Law. Here you will be able to see whether general discussions centre on reform of the Law, or perhaps comparison to another jurisdiction, maybe the Law is working really well and represents a success story of the legal system, etc.
In regard to using this in exams, I would recommend finding a few sources for each key area that can apply flexibly within a topic. Overview arguments, the key points, what someone who knew nothing of the law would absolutely need to know to understand the realities of the Law.
In closed-book exams you will likely not have to reference (at least to a full extent) and so it will be often be enough to say something like 'Smith argues... / Jones states... / Robson suggests ...', or some professors may allow you to just present arguments without any referencing of sources at all.
* This is a key area where you will need to consider this in light of your professors instructions and the specific exam requirements, etc.
Also, just for clarification, this only really applies majorly to university Law students, not so much at A Level.
I hope this helps!
Leah
UEA Law