Validity is determined by the form of the argument alone, and has nothing to do with the premise(s) actually being true. That is, if we assume the premise(s) is true and the conclusion follows necessarily (i.e., cannot not follow), then we have a valid argument.
The validity of any deductive argument can be verified by means of a demonstration or proof. Thus, deduction can yield certainty while induction only gives us probability.
If a deductive argument is determined to be valid, only then do we examine the premises to see if they are true. If they are, we call the argument ‘sound’.