how can i prove that this grammar is ambiguous?

Question

S -> bA|aB
A -> a|aS|bAA
B -> b|bS|aBB

Any easy method other than trying to find a string that would generate two parse trees ?

Can someone please give me a string that can prove this.

Answer 1

There is no easy method for proving a context-free grammar ambiguous -- in fact, the question is undecidable, by reduction to the Post correspondence problem.

Answer 2

There is a string though: bbaaba

S -> bA -> bbAA -> bbaA -> bbaaS -> bbaabA -> bbaaba
S -> bA -> bbAA -> bbaSA -> bbaaBA -> bbaabA -> bbaaba