For verifying the logic, I would iterate through all the combinations of logic 0 and 1. And if you want to check the timings also, then you need to apply the input more precisely and check the output timing is correct. For example, if Q (output) delay is specified as 2.2 ns for 0->1 transition, then you need to check this. Also, you can check to see if it can handle glitches. For VCS, you need to enable transport delay or else the glitch, if less than the gate delay, will be filtered out.