Determining Ambiguity of grammar
If the grammar is not LL(1), can we say that grammar is ambiguous?


Euh, no. A grammar is unambiguous if for every word in its language there exists exactly one derivation-tree for it in the grammar.

Parsing is a way to find that derivation, given the string, preferably in some efficient way. Several classes of grammars were defined that have efficient parsing methods, for example LL(1) grammars.

A grammar can be non-LL(1), and still be unambiguous. For instance when we reverse an LL(1) grammar (that is, take the mirror image of the RHS of every production) the result still is unambiguous: mirroring the original unique derivation trees. But it cannot be parsed in an LL fashion, rather its reverse...