gunbuddy

Hi, How can I prove a certain abelian group is not free? If the given group is small, I can consider all the possible subsets of the group which are candidates to be the basis of the group and prove each of those cannot be the basis. As an example, if I consider the abelian group Z5 with addition, should I consider all the subsets of Z5 and prove that any of the subsets cannot be the basis?