It normally just takes practice. Some tips are:
Employ the IRAC model: your ratio will normal consist of R (the Rule of law) and A (Application of the facts to that rule). So read through the case, and whenever you hit a sentence(s) that sound as if the judge is stating a rule of law (eg a test), that is a good starting point fo ratio. Continue on, where you see that the judge starts analysing the facts of that particular case and applying them to the rule, that can also be ratio.
Also, before you read the case out of the casebook, pick up a corresponding textbook on that unit. There will normally be an explanation of that topic and that very case, in plain English, in the textbook. This will make it easier for you to identify the ratio within the judgement, because you know what your looking for already.
Mind you, its rarely ever clear cut so don't get frustrated if you don't spot it right away. Occasionally remind yourself that your finding judgements hard to read because they ARE actually hard to read for everyone. Judgements rarely have good structure, and are rarely written like anything else that we read, academic or otherwise. Sometimes I wonder whether judges actually learnt sentence structure and punctuation while in school
