The starting point is of course, is checking all the normal credentials like CSLB, insurance and references. But perhaps most important is deciphering whither he is a good craftsman or not. If he is following the industry standards, he is falling way short of good craftsmanship.
Consider that these same industry techniques are what brought you to the point of making this phone call.
Some of the red flags that I see in advertising are as follows:
1) Advertising for a trade that they are not properly licensed for. See "The right contractor’s license".
2) False reviews.
3) False or exaggerated claims, i.e. years in business.
4) Using terms like: "combined years of experience". The aggregate years of experience, of multiple individuals is irrelevant.
5) The most important credential is not how many years, but the quality of craftsmanship or techniques used during the years of experience. If a contractor employ’s poor techniques or a lack of understanding of his craft, the quantity of years are irrelevant.