I know, it is not possible to decide whether a context free grammar is ambiguous or not. However, I am having a really hard time preparing for an upcoming exam in which I'll have to give a prove of un-/ambiguity for a given grammar.
Up until now,
the first thing I'd do is to look for a word (via try and error) which could be build up from more than just one tree. Despite the fact this takes quite some valuable exam-time, it hardly leads to a successful outcome for me. In addition I'm not really sure what to do if it actually is unambiguous.
What would be a proper way to prepare for such a problem without feeling like having to solve a pointless crossword puzzle you can not prepare for?