I guess you are speaking of a closed-shell singlet state? If so, I would recommend you the standard procedure: compute the hessian and make sure there are no negative frequencies. In case you are interested in an open-shell singlet state, then you should consider using td-dft.

Why TD-DFT why not unrestricted DFT?

No need for TD-DFT, use opt and freq keywords with the desired restricted method and basis set. To ensure that you got the lowest energy geometry (optimized geometry), check Frequencies and Convergence Criteria in the output file: there must be no imaginary frequency and the four convergence criteria are met (four YES).

* Using opt=tight or opt=verytight may give improved results.