Do you want to check whether it is represented in MATLAB using the sparse data structure (in which case the issparse function is the correct tool?) Or do you want to check if it is sparsely populated even though it is stored using the full data structure? In the latter case, use the nnz and numel functions to determine the number of non-zero elements and the total number of elements in the matrix. If the number of non-zero elements is much smaller (how much smaller it needs to be is up to you) than the total number of elements, you could call your matrix sparsely populated.
As written your vector is not stored as a sparse matrix according to issparse, nor would I say it is sparsely populated since 75% of the elements are non-zero. A glass that is 3/4 full of liquid and 1/4 full of air is not mostly empty, it's mostly full.
