Your selection of a flight instructor should not be just a philosophical exercise. Every instructor has to meet strict Federal standards to be certified. This does not mean, however, that their skills and knowledge are what they should be. Around any airport with more than one or two CFIs there are usually some whom everyone wonders how in the world they got the certificate bad flying judgment, inadequate knowledge, marginal skills, and terrible personalities. Examiners know who they are, as well.
Instructors have examiners figured out, too.
In what follows, remember that probably no CFI will meet every criterion, so pick those that are most important to you. For example, here in San Diego there have been two instances in recent memory where non-CFIs were giving instruction, meaning signing people off for dual received, written tests, and so forth. If you get instruction from one of these guys and it s discovered, then none that means not any of the dual instruction time from him will count toward your certificate or rating. These people are out there ask for credentials. Flight Instructor certificates expire every two years, as well, so be sure his expiration date is in the future.