Explaining why a grammar is not LL(1)
If the grammar is ambiguous (at least one sentence has more than one parse tree), then the grammar is not in LL(1).

In general, you compute the LL(1) parse table and use it to answer the question. Your grammar is LL(1) if each cell in the table has at most 1 grammar rule. In other words, at least one table entry has more than 1 rule implies that your grammar is not in LL(1): http://en.wikipedia.org/wiki/LL_parser#Constructing_an_LL.281.29_parsing_table