Asking a CFI you approach for their pass fail rate and total hours of dual given would be some basic things to know, and the Gold Seal is another way to show the same. None of those metrics are guaranteed, you can have a mediocre Gold Seal instructor. You could call NAFI and ask them, and the local FAA district office always knows some names. They will give you the most expensive names most likely because they will know who has been around a long time, raising their rates accordingly. Some of the better deals are found at local colleges with flight training departments, they often have new instructors who just need a chance to get started. The most reliable thing would probably be to enroll in a part 141 school with a good reputation, but do some homework on that first. Part 141 takes a lot of the guesswork out of it by teaching a standardized curriculum. Part 61 ad hoc training can be good too, but I would try to find it through a flying club. The least likely method to show consistency would be a local CFI who has a day job, unless they have a really good reputation. I have also seen some strong Part 61 schools run by former airline pilots who lost their medical or just like teaching. Some of those can be excellent.